When you change a flight before you depart, the entire ticket is re-issued, and all segments, including what you are not changing, is re-priced based on availability at the time of the change. It gets re-issued at the lowest fare available, IME, based on the full new itinerary. I don't know if you can specify fare classes online for a change - I don't recall seeing the option - I'd guess it's not there.

If you want to be in W specifically (or above, for example, to be able to use a GPU), I'd call in and request with the specific instructions to the agent to leave the outbound in W (the fare may or may not be availble in W, so if not, you may need to up fare to V, H, or whatever is the lowest available W or above). The 'new fare' total will likely go up compared to what you see online, since W (or whatever higher class) is likely more than T (although depending on fare rules and what you are looking at, it may not be, but usually, it is).