From the Midwest, not used to the heat per say, but were in WDW about a month about or so ago... I know it will be hotter this time, but we did ok with the masks...Hogsmeade and Diagon Alley will be very crowded - always are. Pack your patience in there.
Take lots of breaks if you're not used to being in warmer weather with the mask on.
Not sure how old you are (or when you last came), but the Universal side will look really different to you except for the Hollywood area, around New York, a bit of San Fran, and back by ET. Everything else has changed A LOT over the last 15-20+ years.
Are you staying on property?
I honestly cant tell you the year, but its been a tleast 18-20 since I was there... I think i was in the 10-12 yr old range if I'm not mistaken...
We are staying on property, at the newest "budget" resort at Endless Summers...