What sort of penalties would you think of as reasonable for a swap occuring?
I'll be able to take a closer look on Monday.
At the very least a rank of exhaustion that lasts something like 1d4 hours feels appropriate. Perhaps it also consumes a HD every time inhibiting natural healing.
I'll put some thought into how/when transformations happen.
"Time is an abstract concept created by carbon-based life-forms to monitor their ongoing decay." - Thunderclese