Thanks for the advice mate, much appreciated. The airline are currently allowing us to change flights free of charge, but will hold fire for now and hope that the requirements are dropped in the coming weeks/months.
The picture should become a lot clearer over the next few weeks and by March I'm sure you'll have a much better idea of how realistic April will be. Feb half term will be a big indicator of where the rules are heading.
I'll stick my neck out and say Spain will have quietly dropped the vaccination requirement well before April, and likely revert back to the negative Covid test requirement for anyone 12+ who isn't double/treble jabbed (assuming we haven't all been consumed by another variant)
If your flights are with Easyjet (and I'm guessing they might be.....) just be aware their free change policy only runs up to the end of March. It could well be extended, but equally airlines can't be super flexible forever.
In the unlikely event its getting towards the end of March and we're still none the wiser with Spain, then it might be the right time to take advantage of that free change before the 31st March current deadline.
(if your flights aren't with Easyjet then discard that last paragraph!)