The level of standardization you have as well as the level you want has a direct impact on your migration effort.
For example if your content is currently inconsistent, and you don't care about consistency after your transformation, then the migration is easy. For example, if your data tables each are presented entirely different on each page but you don't care, then migrating is easy. On the other hand, of course if you have high consistency now and want high consistency after the transformation then that is easy, even if you want transformations along the way. The hard part (and it is often worth it!) is when you currently have inconsistency but want to have a more standard approach going forward.
This also has an impact on your inventory effort as well, since if you don't care about the resulting consistency then the inventory may not need to be as accurate for your planning.
Of course, there are many nuances to all this (and the Easy/Hard designation is oversimplified), but hopefully this will help your planning in broad strokes.