I think a bigger issue is why was the 738 downgraded to a 735?

If the 738 had mechanical or inbound was delayed that is OK. Although if your inbound was not delayed - they could have moved your 738 over to another route and left your route delayed - then substituted aircraft.

The bigger issue would be if they swapped your 738 with a route that had a 735 if that route was overbooked and your route was way underbooked.

I had a situation a few years back ABQ to IAH - and aircraft was downgraded from a 733 to 735 and in this case the last row was downgraded - which consisted of a older couple with adult son - likely on paid F.
It appears that the ABQ-EWR was upgraded from a 733 from a 735.

I discussed this with Larry at the DO a couple of years ago and he said they do not swap for load balancing (resulting in downgrades). He was willing to check on this and gave me his card - but I decided not to follow through later.