Some of it could depend on the contract you're locked into.
If you get the right host, you should be able to upgrade or downgrade and receive a charge/credit when doing so. MOST hosts don't do the credit thing when downgrading, so you have to find a special host to do that.
My recommendation is to move to a VPS when the needs require it. Every dollar counts when starting out, so keeping a tight string on the wallet can help.
If you can see yourself growing in the next 30 days to needing the extra space, then it's usually worth upgrading so you don't have to think about things, but again, it'll depend on contracts.
I just recommended to a client last week that he downgrade from his VPS to a shared hosting account. He didn't know why he had a VPS (for the past 4 years) other than it was something someone recommended. Looking at his bandwidth and usage, he's doing less than 20GB bandwidth in a month, and minimal CPU/Memory usage. No reason to be paying for a VPS - yet, GoDaddy has him locked in with no credit for the next 8 months. That's sucky!