Short Engine vs Tall Engine ?
 

Given a replacement short engine, would it be difficult to make it fit with the existing cylinder heads ?

And if not, would it then be possible to replace the cylinder heads later with better ones at a later date without having the whole engine out ?

In each case, even if the capacity of the block hasn't changed, would a remap be necessary ? (keeping the original turbo for the moment).

Trying to budget for upgrades, without going too wild in one step.

Any Impreza turbo head can bolt onto any Impreza turbo engine. You need to allow for compression ratio differences, and choose the gaskets to match the block as well as the required thickness. Don't attempt any of this without talking to a Subaru engine builder.
The intake manifold should go with the heads it came from.

A remap is always necessary if you're going to modify it.
If you can't accept the last one, please sell your Subaru.

Thanks, looks like I'll be saving the pennies for a bit longer, to do it all in one go :-)