installing a backstay adjuster
Tie a sturdy line to your mainsail halyard and then tie the other end to a stern cleat. The halyard will serve as a safety line, to keep your mast from falling.
I don''t see any reason to ease the forestay. When you ease the backstay, that will ease the tension on the forestay. When you get done installing the backstay adjuster, however, you will need to completely re-tune your rig.
Could anything else go wrong? The biggest risk is if your mainsail halyard is old and deteriorated. Also, do you know how to tune a rig with a backstay adjuster, or do you have a friend who knows?