QUESTION - I have the IS4 with everything unlocked and installed except the engine. What happens with any 'exp' sitting available with the IS4 when this thing goes to Tier 10? Will that become free exp?
Ha, you wish!
What will happen is that IS-4 will get moved to tier X with the best gun and it will be elite (the gun is the only upgrade available).
You will get the IS-8 with a garage slot and a new 100% crew, it will have everything but the engine researched and the xp that you have currently sitting on the IS-4 will now be sitting on the IS-8.
I'm pretty sure the modules that you have upgraded on the IS-4 will translate to the equivalent modules on the IS-8 being upgraded. i.e. if you have the upgraded suspension on the IS-4 then the IS-8 should have the upgraded suspension on it already.