How to modify your old bathroom ?

Modifying your old bathroom means making changes to it.

Yes

What are you trying to modify it for? Change it into?

We modded one bathroom into a darkroom and modded another more recently to deal with a partially disabled person.

So, what are the mods trying to accomplish?

If you're just looking to go to a composting toilet, you remove the toilet and cap off the drain. Then just install whatever kind of composting toilet where the old toilet was. Some of them need special venting, so you'd have to figure that out.