+1000 that you should not have told the guitarist in band A that they would take priority. First come, first served. Honor your agreement on existing gigs but tell them you're changing policy for the future.
"Moving" a gig IS canceling a gig. It's just canceling one gig while trying to book another. But the original date is still getting canceled. It still leaves the venue with a hole to fill in the schedule and the band with a night not working.
Although you are the contact who does the booking and talks to the bar, decisions that affect the band should be discussed with the band first. I would have gone that route.
If, indeed, you provide the practice space, PA, and do the bookings for band A, seems to me you are band A's default BL. The others are just showing up and playing. Seems to me a talk is in order about the band's policies and decision-making. On the one hand, if you step up and claim the BL role, that entitles you to more proactive decision-making. But at the same time, a good BL needs to think about how to manage his musicians to keep them happy and contributing.
I had a guy like this guitarist once, maybe not so bad, or maybe it was just that it never got to that point. I joined this group via craigslist while already trying to get another project going with a singer-keys player. It had never come up that I was doing the other project and neither group was gigging (never even got any other musicians to complete the keys player's band). Was in a music store with my wife and ran into him, and introduced him as "the guitarist in the Manchester band." This expression of shock comes over his face and he says, "You mean the guitarist in THE band." Like since I had joined his band, it was offensive that I might play with anyone else. He was a long-time bedroom player and this was the first time he had ever played in a band of any kind.
The reality is that a "band" is just a group of musicians playing together. You have to do right by anyone you make a commitment to, but this whole jealous/possessive thing like "since you play with me, you're not allowed to play with anyone else," as though they owned your time, is absurd and should be corrected ASAP.