Actually, from beancount's perspective, selling the old fund and buying the new one is what you want to do, I think.
The trick is that you need to specify the cost basis and the acquisition date on the new fund so that there aren't any capital gains. The way you do that is with the {...} syntax.
2021-09-16 "Merge to new fund"
Assets:Brokerage -100 OLD_FUND {11.00 USD, 2015-05-12}
Assets:Brokerage 200 NEW_FUND {5.50 USD, 2015-05-12}
Your broker should able to tell you what the cost basis in the fund is. Give that a try and if you run into problems post here again about so people can help you out.
Cheers,
Justus