Full to Simple recovery mode during massive data update?
The only thing you're really changing by using simple recovery is you're shifting the time when you have to perform additional backup work. Instead of taking log backups throughout the process (which shouldn't be extra work, because you should already be doing that!), now you have to take a full backup at the end of the process. In the meantime, you've opened yourself up to real problems because should anything go wrong between the start of the operation and the completion of the full backup, you are going to be in some hot water because you've lost the ability to recover to a point in time.

(Since the bulk of your activity is delete, you can't even benefit from minimal logging - and even if you could, Thomas' advice to use bulk-logged is a better approach than switching to simple - this still allows you to recover to a point in time that has been backed up by the log and is not inside a minimally-logged transaction.)

As you're planning to delete 90% of your data, you may want to think about this in a different way: move the data you want to keep to a new table, then drop the old table and rename the new one. The stuff that costs a lot of log is when there is a lot of data movement. DROP is logged too, but unlike DELETE, only the deallocation, no movement. You'll need a little extra space to support this (~10%) but you'll also avoid any after-the-fact rebuild.

All of these options involve trade-offs, but I think your plan to change recovery to simple offers the least benefit and introduces you to the most risk. I wouldn't do it without having your resume dusted off.