You are mixing up terminologies here.

I think you are referring to trim?

You can cut back siding and install a new trim. It will require someone with some carpentry skills as well as trim forming (i.e. trim brake).

 

Yes, that was my original thought. We discussed with the siding contractor and he was going to use a 3" j-channel to give the effect of a wider window trim. Now that I'm keeping the siding, I'm looking for the easiest way to achieve a wider window trim. I know using the wide j-channel is sort of the chintzy way to do it but if it saves me a good deal of money, we'll be alright with the look. Now I'm not sure how in the world I would secure a wide j-channel to the house because I can't nail behind the siding...with the siding in place.

I guess my question should be that: can I retrofit a 3" j-channel to look like wide trim or will I have to cut back the siding, lay on 1X4 trim and have a siding guy bend some aluminum to fit?

 

I just found exactly the product I need. 3 1/2" snap on lineal trim. It snaps onto 1" j-channel to extend the trim. See attached photo. Anyone ever hear of this before?
It's even listed on Menards (not that there are any near me)
https://www.menards.com/main/building-materials/siding/vinyl-siding/vinyl-siding-accessories/3-1-2-snap-on-lineal/p-1444447310523.htm

 

Never used it but am a worrier and just a couple wrongs that might happen.
1. Wider trim that can bend or warp, basically ruining the flat stock look you want.
2. Without some glue or tape, strong wind/rain might catch it and rip it off. Many things, leaves for example, can catch and may be difficult to clean if you can't reach.
3. Your original siding must be perfectly flashed since the cover will hide all failures.