Downtime when upgrading Basic/standard to Premium tier

Hi,

One of our customers is currently using basic and standard tier SB namespaces.
As they were recently impacted by a problem in the backend which caused about a day of downtime, they are investigating if they want to start using the Premium tier.

The question they now have is if switching to the premium tier would involve downtime and if so, for how long?
I'd expect there to be some downtime as you're moving from shared to dedicated infrastructure (as I understood), but wasn't able to find a definitive answer.

Thanks in advance for any info!

Paul