>>Does anyone know how difficult it is to replace and what the pitfalls are?

A qualified Roland tech can replace about anything on a synth; the question is generally not whether they can do it, but whether you want to spend the money. If Aftertouch is important to you, go for it. Life is now.

>>>Anyone know how to access this function?

Patch Edit -> Control -> Aftertouch

If you have a manual, check the index; it lists all the pages where Aftertouch is covered.