Better to call Transat directly, and ask if your existing voucher is treated as “cash” in reserving the new flights for next year. And if booked under the ‘Peace of Mind’ promo, and paid for in full then cancelled, if your tickets would be eligible for a new voucher to be issued. I strongly suspect that this would be the case, and should reassure you about rebooking now.
The airfares for travel to Europe in 2021 with Transat are indeed looking very reasonable. If you’re hoping to go, you have nothing to lose by booking your tix now. Plus, you’ve got the best availability for reserving the few two-two seats config in the tail, and the possibility of reasonably priced Club seats as well.
Your only risk is to maybe see some sched change activity on your flights between now and next year. I’d look for fully cancellable accommodations for your new dates next year, and pretty much hold off any other trip booking that would have cancellation penalties until the last minute.
My travel agent was surprised at the good deal that I snagged for Club seats to Portugal, when I recently got her to book them for me for early next year. I had her call them to double-check the questions I had about the booking conditions, before we proceeded. I deal with my local CAA branch, whenever I want a solid travel agency at my back. Since your Transat voucher is like cash, and travel next year can be complicated, I’d strongly recommend that you make your next Transat booking through a local agency - and automatically come under the OPC/Quebec or the TICO/Ont provincial consumer protection rules (depending on where you live.)