If nobody else responds fully (I've not tried this):
Google mail at this end is a set of DNS entries. Before switching to Forwarding, be sure to click the DNS link for your Fully Hosted domain and note all of the settings for the mail entries: MX records and A records.
Once you switch to a Redirect, click the DNS link again and see if those settings were retained. If not, then re-enter them. And if it totally blows up, contact Support. You won't lose any of your email, but some of it may bounce while it's broken.