I've noticed after a 2 hour rental of irons that if they aren't removed before the end of the rental period and replaced with your previous set, that the irons section of your equipment disappears from the edit equipment interface and there's no way to re-equip your original clubs in the PCEA version (the Mac version from Steam in my case). This has happened with the last two sets I've rented.
The only way I could replace my original clubs was to open the game through Chrome's Flash functionality and replace the clubs in the edit equipment section there. The same thing does not happen when replacing optional clubs, just the required irons section. Have not tried with other required club categories.