That's a possibility. But, Fresno already got bumped down because of three things: location, location, location.
What I mean by that old real estate mantra is that three aspects of the location were considered deficient for Fresno at the Triple-A level.
First, a big part of the reason for the minor league realignment was to get major league teams' higher level affiliates significantly closer to the parent club, instead of having their Triple-A team three time zones away, as had been the case in recent years with the Mets being paired with Vegas and Fresno being affiliated with Washington. Essentially, the distribution of Triple-A teams across the country was too out of line with the distribution of MLB teams. Realignment of teams not just by league but also by level was the fix for that, accomplished by bumping some lower level teams up, like Jacksonville becoming the Marlins' Triple-A affiliate (and even a couple of indie teams joining affiliated ball). That however meant that some existing Triple-A teams got bumped down and Fresno was one. If the A's move to Vegas, then the distribution wouldn't be a problem, as it would still be a western MLB team with a western Triple-A team, just with Vegas at the major league level and the Triple-A affiliate in California instead of the other way around.
The second location issue is the ballpark itself. One of the other major reasons given for the realignment was to get affiliated teams out of ballparks that were in poor shape and lacked the level of athletic facilities the parent clubs wanted their prospects to have access to. Fresno's ballpark is not bad, being it's still in affiliated ball, but it is supposedly below the new standards set for Triple-A, so any return to Triple-A would require some upgrades.
But the third location issue, and probably the biggest one that Fresno faced and still faces is that Fresno has few direct commercial flights. While Triple-A teams do fly, it's on commercial flights, not charter flights like major league teams. So having a paucity of direct commercial flights to Fresno, especially from other Triple-A West cities, makes it undesirable as a Triple-A city. It was already an issue for years. Conversely, the lower tiers travel by charter bus. In that regard Fresno fits in with a lower level much better. And there's pretty much nothing the team can do about that. They can't magically make airlines add direct flights. And they can't be allowed to travel by charter flight when all the other teams in Triple-A fly commercial, as that would be an unfair advantage.
So, ultimately, it might be infeasible for Fresno to return to Triple-A.
(The other question is if Fresno does go Triple-A, where does the Low-A affiliation wind up? It'd have to be somewhere else in California as that's where the entire Low-A West league is located. A possible location is Lancaster, where the team was dropped for Fresno in the reorganization.)