I'm afraid there may not be a way to do this since we're not load balancing, but I'd like to know if there is any way to upgrade an EC2 EBS backed instance to a larger size without downtime.

First of all, we have everything on one instance: both our app and our database (mongodb).

This is along the lines i'm thinking: I know you can create snapshots of your EBS and an AMI of your instance. We already have an AMI and we create hourly snapshots. If I spin up a new separate instance of a larger size and then implement (not sure what the right term is here) the snapshots so that our database is up to date, then I could switch the A record of our domain from the old ip address to the new one. However, I'm afraid that after copying over the data from the snapshot, by the time it takes to change the A record and have that change propagate, the data could potentially be stale. Is there a way to prevent this, and is there a better way to do this than I am suggesting?