<<should i change the baggage with in the said dimensions.>>
I feel that this is always the best course of action. Not only would I keep the "linear aggregate dimensions" to, or < those specified, I would also make sure that each dimension complies with those of the airlines, including all handles, wheels, etc.. Then, I would be sure to keep the weight within the max. specified, if there is a weight restriction.
When UA changed their carry-on dimensions a few years back, we bought a new case, just to meet their new specs., though we had one that was just barely over. We just went with a newer case from Tumi, which DID match UA's dimensions perfectly, and with Tumi, unlike some mfgrs., the dimensions stated DO include every aspect of the case - handles, wheels, etc. So far, that case has fit perfectly into the sizers, with zero questions.
Good luck and travel safely,
Hunt