How to do a rain backup plan?

Just rent a tent, just in case

Unless your venue has another room to host a cocktail area in, it's not really feasible to "flip the room." That would take awhile (if your venue is even willing).

I would say your options are 1) a tent, 2) do the ceremony while people are seated at their tables in the reception area or 3) a different venue for the ceremony only.

What kind of venue are you using? Every venue I looked at specifically told me upfront that they already had a backup plan in place for brides in case of rain. Just curious, if your venue has done weddings before why they wouldn't already have an idea of how to make something work.

We're also planning an outside ceremony, but in the event of rain to save time and money guests will sit at the reception tables, we'll create an aisle between them and get married in the reception room. It's less work for the venue, which means we pay less and also gives us up to about 20 minutes before the ceremony is due to start to decide whether an outdoor ceremony can be pulled off. I wasn't keen on the idea at first, but the caterer really sold it well!

My venue offered a backup plan if it rains. Are you absolutely positive the can't accommodate you and your guests if it rains? Maybe have a conversation with them and tell them your concerns I'm sure they can come up with something.