TL;DR: today's change has retroactively changed the meaning of our past voting, and I'm uncomfortable with that. I want to point that out, as I feel it's a concern that's not been raised so far. I am also looking for input and any kind of solution, if someone thinks of one.
A couple of things I feel should be said first, but am unsure how to introduce:
- retroactive unlocking of votes, especially upvotes, following today's changes, sounds technically hard and I don't expect it to happen. This is more about the philosophy of voting;
- I very well know of "vote for the content, not the user", but this is not that binary here; once again, this is about the idea of the vote itself, not "just the content" or "just the user".
As of today, question upvotes are retroactively going from +5 rep to +10; whether it's a good idea or not is not the matter of this question. I, and I guess other users as well, will now be less likely to cast upvotes. It is not so much about "voting for the user" now that the up arrow gives 5 more rep, but rather what it means to upvote stuff.
If I upvote a question, it means I want to see more questions like that. Over time (and especially on Stack Overflow, I guess), one's threshold for quality might change, and the user gives upvotes more or less easily. In the latter case, they might end up wanting to retract some earlier upvotes, but can't because of vote locking. This is fine; their own policy, their responsibility, their consequences to assume.
But here, the "threshold for quality" was imposed by a third party. Today's change didn't just trigger rep recalcs, today's change has retroactively changed the meaning of my past voting. And that's not something I'm comfortable with if I'm not given the possibility to reconsider my vote.
It's not a perfect example, but a real-life analogy might be:
In 2017, I voted for Mr A., who had campaign promises X, Y, and Z, to become mayor of my city for 4 years.
Today, the country passed a law saying mayors are now elected for 8 years, effective immediately and retroactively.
Mr A. is still Mr A., his ideas are still X, Y, and Z, but I voted for him to be mayor for 4 years, not 8. Had I known it would be 8 years, I may have voted differently. I don't want the word to be put in my mouth that "yes, the reasoning behind my 2017 vote is in line with the 2019 change".
This is not really a feature-request (though I'd welcome it!) because I think it's technically too hard to track and implement. I'm rather looking for input on does me changing my upvote habits because of
this change the quality I now deem the question to meet, with a new "feel" upon voting, means I am now prioritizing the user instead of the content? (I don't think so, but maybe I'm missing something) as well as pointing out (again) that voting is a kind of contract, it engaged you (even if it's anonymous), and retroactively changing any effect of this contract should put it eligible for retraction. Should we be allowed to change our previous votes, in regard of the new philosophy of upvotes?
As I commented on the answers below, I'm afraid I didn't word the question clearly enough. I'm not concerned by "having given someone 5 more points". I'm concerned about the comparative threshold above which I'll upvote. While we have guidelines for votes, they're votes: part of them remain tied to a feeling; at least the "useful" upvote indicator seems to be ("not only it's a clear, on-topic question, it's a question I feel is useful").
In the long run, questions that I now upvote may not actually have more value than previous ones, but they will certainly feel that way.
Now that questions are now to be considered of the same value for the site as answers (which I'd say are always more useful to the site), that feeling part has evolved and I'll judge questions and answers the same way. Thus, the past voting that is attributed to me was not cast with said feeling.
I have difficulty (if that's not obvious) of conveying that what's making me uneasy is not the increase in rep. Thanks to Jon Ericson's comment I'm getting closer to manage to word it right: it's saying I voted for something while changing this voting's parameters. Had this been always only designated as a score I don't think I'd have reacted the same.
I realized I'd have felt the same way if the rep change was halved, or if it displayed posts I'd upvoted with a green background, or really any kind of (not a specific) side effect. Words matter - voting, for me, has an especially important meaning; so I reckon this is a me problem, not a SE problem. As this wasn't obvious to me, and possibly to others with the same wiring, at first, I think this post can serve to put words on it.