insteon is a bit quirky, as you are probably aware. then there are few additional quirks in terms of how the HA insteon integration works.
there are multiple types of devices you could be using a central controller including insteon PLM module (e.g. 2413U but there are a few other variants). or you could be using an insteon "hub". this is a mini computer/webserver with an integral powerline modem. you mentioned PLM in one of your posts above, so i'll assume you have something like a 2413U.
here are general roadmaps you could take:
plan a:
remove PLM device from homeseer. you can power-cycle it but do not reset it. connect it to your home-assistant server resulting in "all-at-once" migration.
home-assistant (and insteon integration) should auto-detect the presence of the modem, then will connect to it, then should autodetect your population of insteon devices that are already linked with the PLM.
big advantages of this approach: 1. all of your insteon devices were previously configured to talk to your PLM before. their basic configs won't change. thus you don't have to go all around your house to all your devices and physically access your devices to reset them and/or re-link them. 2. you won't have any problems related to startup with sparse mesh network, phase bridging, etc.
some disadvantages: the initial setup and auto-discovery is a slow (multi-hour, in my case at least) and opaque process. it is pretty daunting to do if you are learning home-assistant, the insteon integration, etc. all at the same time.
plan b:
leave insteon in place controlled by homeseer and gradually migrate to a (separate) home-assistant insteon system.
leave your working system on homeseer in place. leave the PLM device attached to homeseer. purchase (e-bay?) another insteon PLM and attach it to your home-assistant server. one at a time, take single devices or small subsets of your devices and delete them from your homeseer config, then factory-reset the devices, then re-link them to the home-assistant insteon controller. your homeseer network will gradually shrink. your home-assistant network will gradually grow.
advantages: 1. all your stuff in home-assistant will be clean and reset (no old/broken/forgotten links or other cruft hanging around). 2. start simple. test as you go. learn to use/control home-assistant and insteon integration starting with a small set of devices. weed out dead/unreliable/flaky devices as you go. 3. reduce disruptions in your house operation, your lights, your existing automations - since your original system mostly stays in place, at least initially.
disadvantages: 1. more work overall. each device has to be physically manipulated. 2. insteon is a dual-band (wireless/powerline) mesh network. you may experience situations where starting with just one device will not give you robust comms. therefore you sometimes need to be "strategic" in terms of picking the first few devices to migrate. 3. requires that you purchase more gear (an additional insteon PLM), but otoh, when you finally complete your slow migration, you'll end up having a backup.
other general caveats:
oldest-generation insteon devices actually use a version of insteon protocol that is a bit less robust. devices can either fail to work or often they'll work for basic commands but when you go to do something more complicated (like re-linking them or editing links to other devices), you'll experience problems.
insteon was originally a powerline only protocol, then evolved to be a dual-band where there are two parallel mesh networks with multiple bridge-points - one powerline-based and one wireless. so its good to be mindful in general whether you're dealing with powerline-only devices (typically older hardware), dual-band devices, or wireless-only devices (typically sensors).
the powerline part of the insteon network has an additional quirk that the two separate powerline phases of a normal (for the US, at least) "split-phase" power system are partitioned and typically can't pass powerline signals. ...or sometimes they can, but with greatly reduced S/N since the signal path goes outside your house to the nearest transformer and back. fortunately, the fix for this is pretty easy if you have multiple dual-band devices on your network and at least one or two on each of the two phases, then you'll have effectively "bridged" your two phases. but (particularly with plan b above) you may not reach this happy place until after you've succeeded in linking several devices across both phases.
if your insteon system was reliable and working well under homeseer, chances are it will migrate fairly smoothly and will then work fairly well within home-assistant. if you have any troublesome/unreliable devices, incorrectly-linked devices, devices reporting low-battery, devices that don't provide healthy heartbeats, etc. i strongly recommend you diagnose and fix (or replace?) before starting the migration process, because multi-fault diagnosis can be hard in the best of cases, but is harder still when you're working with unfamiliar tools and climbing the learning-curve.
docs on the insteon integration (and underlying pyinsteon package) are pretty sparse. many topics are simply not touched on at all, so you have to learn by experimentation or by mining forums for clues. good news: it's all open source so you can always fall back to reading (python) source code.
PS: this is getting to be a wide-ranging/rambling thread and as we discussed, was marked as "solved" months ago, so is probably being mostly ignored by others - i suggest if you hit snags or have specific questions, you open new specific threads with descriptive titles/tags.