afaik there is not an official method which would facilitate a swap meaning there is an inherent risk in doing so. I don't know how you would swap and be 100% sure the tickets were legit and also still valid (there are rumours those sold on secondhand market will be invalidated) which is unfortunate. But I also agree with you that if you sell the ones you currently hold then your bargaining power is gone and likelihood of attending lowered, even if selling then repurchasing from TM or AXS is safer.
There are FB groups fans have set up for each of the nights/cities and if you join yours you might find people happy to swap - many fans had a preference for standing over seated (though it is a long show and while I was happy to stand for Rep it wasn't the case with Eras). You would probably want to engineer it in a way so that you met at the doors and went in together to reduce the risk. Bear in mind that your VIP tickets grant access to the stadium 1 hour earlier giving those ticket holder a chance of getting the best spot. If I was swapping from seated I would want to take full advantage of that.
I think there are similar swiftie reddit/discord groups designed to try and help as many 'true' fans go as possible, you might find luck there.
Who do you know in person who is going and trustworthy? Even if it's a swap for a different night it could be your best bet if your cousin, for example, is going with a group of adult friends? I would make sure you knew how they got their tickets, you don't want to swap with someone you trust only to find they had unknowingly fallen victim to a scam.
You might get people saying to just doll up the 12yo and have them pass for 14, and that might work, you could keep an eye on how that goes for the Wembley dates in June. But it's a huge risk, you can't prove they are 14 when they aren't and them being turned away at the gates would be devastating. And these rules are in place for their safety (plus 12yo are shorter than most adults so their view would be restricted).
Honestly, good luck, but be careful.