Quite a few sites I visit have been upgrading and switching servers lately, which is promising to look forward to bigger and faster servers, but they always seem to end up with numerous issues including and up to deleted accounts and several days of downtime. What are some common issues to look out for when switching servers, and are there any ways to get around them?

Common issues would be server configurations not matching. Make sure if you have something installed on the old server, it is installed on the new server.

When moving accounts the host should also double check everything has moved over properly.

Depends on the type of hosting.

One fairly common issue is making sure the old and new servers have the same versions of software, or if software is to be upgraded too, making sure there are no compatibility problems with client websites, etc. Configs need to be transferred across too.

The new server should be able to take over the old IP address(es) when the move happens.

Downtime's an interesting one. One great thing about VPS is that with the right technology, and old and new servers running side by side, clients can be live migrated and no downtime is seen. Shared hosting can benefit from this too if running on a VPS. I guess it's hard to be so seamless with dedicated servers but it still shouldn't take several days.

I think the one with several days of downtime may be running their own server (albeit with less than spectacular results) which could have lead to or been the problem when they tried to switch to a new server. I'm kind of afraid to ask and see exactly how clueless they are.

Its always interesting to se sites that switch without you even knowing and then the ones that seem to switch and crumble at the edges.

I always recommend testing a lot when you switch a site. What I do when moving a site to a new server is copy the site over (keeping it live on the old server) and then edit my hosts file on my PC to point the domain to the new IP address for my browser only. I then test to make sure all the scripts and stuff works. If all good, I then make the DNS switch and 24 / 48 hours later I suspend and then delete the old account.

As Matthew said, you should really get the new site up and running before you make the switch. Other things to be sure of would be to have back ups of all forums and member information at the last possible minute. Most membership losses are caused by not having recent back ups, and so the newest accounts are simply not copied over.

The number one thing people forget to do is"back up your data prior to a move"
Its your data so back it up!

This is a VERY important step. Even though most hosts will back up your data for you it is still your responsibility to keep data safe and your own copy should things go wrong (which they do from time to time).

Having completed many migrations and some of them have been flawless and some of them have been complete train wrecks. Here is a list of issues to be focused on.

1. Make a complete backup of all of the data.
2. Communicate with the client about the migration, time, new passwords etc.
3. Understand any special settings for the specific accounts.
4. Make sure you have access to either the accounts DNS and or their domain name to make the DNS changes without any delays.
5. Do not switch DNS on more accounts than you can handle.

I hope this helps.

SW license issue if you have clients. They are not patient when SW license is not ready.
and IP and DNS is stressfull too.
Sometimes, it takes so long to complete server move.