Whiskey,
I doubt there will be a long-term effect, as we already have a number of planes effective at higher altitudes (Ta-152, the P-51, P-47s in general, hell most of the later American Iron) but that hasn't moved fights up.
Fight altitudes are always going to be dependent on the map far more than the plane set. Maps with bases further away just naturally drive things higher, while closer bases concentrate things down low. And I was taking off a base further back for a climb to 15-18k even before the M (looking out for high P-51s entering the fight). Even with the planes we had, most cases all you found at 20k were a few Ponies and Typhoons/Tempests looking for an easy pick 10,000ft below them. Even BUFFs rarely fly above 15k in the Mains.
Incidentally, of the Ms I've seen almost all were entering the fight under 10k. However that may have just been the results of the local theater I was fighting in (A19-A1 furball in LWB on Uterus, which is generally a low-alt fight anyway).