For the purposes of general information and because this arises with some frequency, the EC has issued guidance (which does not have the force of law, but likely comes pretty close) which, as one might expect, now makes clear that EC 261/2004 downgrade compensation is 75% of the value of the ticket for the segment which has been downgraded and not for the entire ticket.
This is due to you in cash (or cash equivalent, e.g., bank transfer, prepaid card). You are free to accept a voucher, but if you do, it ought to be for more than the cash amount as you are losing flexibility.
A strong suggestion that you deal with rebooking issues with the various agents and then deal with EC 261/2004 compensation issues after your journey is complete, you are back home and can make the claim through the provided channels. The lone exception would be if the proper amount is offered in the proper form to you on the spot.