In order to avoid realizing some of the losses in their funds, many companies are asking if they were to leave their current provider and trustee and go with a new provider and trustee, can the mutual funds move in-kind. This is not a brokerage type arrangement.

Is it possible to transfer plan assets in-kind between trustees?

I assume you're talking about a corporate trustee, since individual trustees change regularly with no impact on the investments. Each service provider is likely to have their own rules for this.

It all depends on the investment arrangement. Certainly, funds can be transferred from one brokerage account to another, unless they are proprietary funds. But if the plan is using a platform, the new platform may or may not support the old funds.

In any event, the concern about selling funds "at a loss" is misguided and should not be a factor. If you sell a fund or funds at a low price, and turn around and buy the same or similar funds at a similarly low price, there's no harm done.