Would the 753 be able to operate out of every airport they serve though? The 753 needs an 8000ft+ runway, which is about 1500ft more than a 752. They'd probably need to keep a few 737s, but I think most will be brought to the mainland.
It seems to me that it would be smarter to reconfigure the 753s to have fewer F seats. The 738s have 14F, but the 753 in its current config is 24F. That's a pretty big increase. It would probably be smarter to remove two rows of F and replace them with 3-4 rows of Y. I'm assuming they'll remove E+ as well? The rest of the Guam fleet lacks E+, which kind of sucks. It would be a nice upgrade for the Guam fleet though...the 753s are nice, new-looking aircraft, whereas the Guam 737s I've flown on felt kind of neglected and worn out.