Okay, I’m going to be the jerk here...
Assuming this is a volunteer situation, this is simply unacceptable. If your worship band is made up primarily of volunteers and the worship leader paid in any capacity by the church, this is way too much to ask. However, if you guys are all paid then this just may be the way this gig is with this particular WL.
Is it possible to talk to the WL privately and voice these concerns? Frankly, learning a group of songs and then changing up the songs at the last minute is way too much to ask of anyone not paid the local market rate, and it sure sounds like the WL is taking advantage of the worship band. I could be wrong though, and maybe this is the way that churches function where you live, but if this were me I’d be having a serious “come to Jesus” y’all with the WL about this blatant disregard for your time and preparation.
When I played at our congregation, it was about one service a month, and just one song at that service. Each month, the song would change.
The music coordinator was a paid position and the musicians all volunteered. The reason we only played once a month was that that was the best way to be fair with volunteers and be respectful of their weekend time. And even at that, if a musician couldn’t make it, it was cool. And if someone had trouble learning the material, or was a beginner, that was cool as well. The group was formed so anyone would feel welcome, and never pressured.
OTOH, there was a pianist that played 4 services a weekend, every weekend. And he had to be an accomplished musician as he was leading the hymns as well. That was a paid gig for him, the pianist was a music teacher at a local high school, and this was his weekend job to supplement his low teacher pay. He played approx 4 songs, the opening and closing were always the standard opener and closer, and two songs in the middle were new each week.
In addition to that, during summer, the paid pianist and volunteer band were cut loose and it was just the music coordinator who would perform at just one service a month. Again, this was to be respectful of people’s weekend time and vacations.
Never was there any pressure or guilt tactics used to compel any volunteer to show up and perform. We were all adults, and knew what was expected, yet as adults, we also knew it wasn’t a perfect world and s#%t happens.
To the OP: Sounds like your band is all volunteer. Don’t get all worked up over it. If it ceases to be enjoyable, there is no risk whatsoever to stop.