Yes, in both directions, and you are never locked into the plan you started on.
Upgrading within shared hosting is immediate. Request it in the client area, pay the prorated difference for the remainder of your billing cycle, and the new limits apply within minutes. Nothing moves, nothing goes offline, and you do not need to do anything to your site.
Moving from shared to VPS or dedicated involves an actual migration to different hardware. We schedule it, copy everything across, let you test, and then switch. Downtime is usually a few minutes at most, and we can do it outside your busy hours.
Downgrading works too, with one thing to check first: your current usage has to fit inside the smaller plan. If you are using 40 GB and the smaller plan allows 20, clear space before the change rather than after.
Billing is prorated in both directions, so you pay for what you use rather than for what you originally guessed.
Timing advice: upgrade when your resource usage is consistently near its ceiling, not when it spikes once. A single busy day is not a signal - a busy fortnight is.
