Actually, I think you always have the right to change your mind.
I don’t think you should have to volunteer when you were pressured into it and you caved in. So what?
People like this woman really bug me. You said ‘no’ and that was not enough for her. I would call her and say “I’ve been thinking about it and I really do not want to volunteer on this occasion so I am backing out”. She’ll probably be mad. So what? Who wants to be friends with someone who pressures you into doing things you don’t want to do?
You could possibly say to her “I will be glad to help you out beforehand if you need me, but I do have plans at the gym for that night”. And maybe you can offer to help out after your workout if you feel like it. But you shouldn’t feel bad because you prefer to work out. You don’t need other people’s approval to do what you like to do.
I think you haven’t been in the position of trying to get enough volunteers to staff a parish project. Many of people in that position bend over backwards to try not to pressure anyone, but many others find that a little push to get people out of their comfort zone can lead to a volunteer who was glad they went beyond what they were originally willing to do.
As for “She’ll probably be mad. So what? Who wants to be friends with someone who pressures you into doing things you don’t want to do?” this is not just about thinking about what this is going to do to your social life. It is about not letting down someone whom you told could rely on you. That is called integrity.
Now, Angie could have called the woman the next day and said, “I didn’t want to have a confrontation in front of everyone, so I just went along with you, but I’m here to tell you I am NOT going to be there and you had better NOT pull a stunt like that again. That was unacceptable, you know how I feel, so don’t do it again.” It would be OK to express anger over such a blunt contradiction of the initial refusal and it would be OK to not be too concerned if the woman was not happy to get the correction.
Your idea about calling this woman and asking if she could be flexible about time because you would like to slide your schedule around, however, is a good one. Many people do that, and as long as you are willing to get “no” for an answer, it doesn’t hurt to ask. Sometimes, the organizer finds that the slot you took was the easy one to fill whereas other slots are proving hard to fill. Check in about whether changing times works for the organizer. It can’t hurt to ask, provided you don’t pull your promised contribution without having really been forced by circumstance to do it. Again: That is about having integrity, not about trying to get other people to like you.
Organizers of projects staffed by volunteers ought to take note, however: People you pushed into signing up to help are far more likely to flake out on you. That is predictable. It is tempting to push, but don’t do it. People will resent you, both the people there who really don’t want to be there and the people who are left running your project short-handed because so many of your volunteers let their resentment become an excuse to leave you in the lurch. The next time you try to get volunteers, people will run the other way when they see you coming.
I’m not even sure what that means, but I guess that isn’t really important. If you tell her now that you don’t want to go to the fundraiser, will she have enough time to find another volunteer?