There are side effects while switching the VDDL between 2.5V and 3.3V. We have default output drive settings for 2.5V (=M High) and 3.3V (=M Low). If the device is programmed for 2.5V outputs, when VDDL is powered with 3.3V, the outputs will be driven a little harder with faster edge rates. The reverse will be true as well, if the outputs are programmed for 3.3V, then when VDDL=2.5V the outputs will be driven a little weaker and edges will be slowed down.
The CY22393/4/5 datasheet specifications on edge rates and IOH/IOL are based on the default output drive settings.
Jitter may or may not be affected. Increased edge rates could help lower jitter due to the increased rate through the threshold region, but then again you could have increased noise due to the higher edge rate which could increase jitter as well.