What happens to the guild stash tabs if I join a different one?

Right now it's just me and a couple of friends in our guild, but we are considering joining a larger group.

What would happen to the money we've spent on premium stash tabs in there? Are they just voided?

yea. if no one is in the old clan its voided. (no one is chief, no one
can invite anyone new. also all tabs and items will be lost.

solution: create a new account (for free) buy nothing. join the clan
and be appointed the chief by the current chief, and just dont use
it anymore unless you break up with the new clan you've joined.

you can store excess items in standard in the old clan, while being mostly
invested in the new one. it could be a year from now you and your bros bail
out on the new clan, and all you gotta do is invite your crew back to
the old clan with that account you made chief and never really used!

lvl 1 clan chief with no mtx and no stash tabs ftw.

If you want not to waste your bought aditional stash tabs, send a mail to the support, they can merge the tabs from the old guild to the new one.
But this have to be done by the guildleader (old) and the guildleader from your new guild have to confirm that this is ok.
support will you give detailed informations, how to do this.
I did this about 2 years ago, when my own guild 'dies' and i joined another guild

Thank you both for responding. That answers my question perfectly.