No, you don't forfeit your rights.
They notified you, so you can now cancel your travel (if the delay makes it no longer useful, full refund) or just, now you are informed, so you can plan the arrival better (e.g. cancelling or notify the hotel, transport, etc.). I really like when the airline notifies me in advance.
If you accept, you are still entitled to compensation. Or you can phone them, and ask for an alternate arrangement (in this case you will not get compensation, but may if you negotiate). By phone, you may also negotiate more compensation, maybe lounge access at the transfer airport, if you have a long layover. But make it clear that this is extra to the compensation. Be nice. On the phone (and email) you have just employees which are not responsible to the delay, and they could just become "robots" with their answers, if you are aggressive. So by being nice, you may get more willingness from their part, to help you further (unfortunately, this depends also on previous calls [not by you]).
If you don't confirm, there is risk that they think you will not travel, so you have more probability to be one of the overbooked passengers, so eventually more delays can occur (but after 8.5h delays, there is no additional compensation, just compensation for long delays), but for the care [hotel, food they need to provide you].
So I would phone, and tell them I'm not totally happy, and I would ask for a way to make the travel better, but I'll still ask for compensation (but if the extra perks they give me are more valuable).
EDIT: some more information
Note: It is August 2020, and you got a notice only 3 days before the travel. So for me, this is just an operational reason, nothing about last minute emergency or COVID uncertainty. Airlines probably will argue differently, but just keeping arguing, they just try to filter out the less motivated people.
Regulations https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:32004R0261&from=EN (sorry, the HTML format sucks a lot).
Article 5 tells you about the right of compensation. You have a reroute, but you were informed less than 7 days before departure (and you will arrive later then 2 hours after your booked flight).
Article 15 is about the waiver, so just by confirming you doesn't waive anything. They should really make clear your rights and that you don't waiver your rights (e.g. with extra perks). This is not the case with normal notice of delay.