I have finally found a new hosting company for my websites, but now I need to transfer them over.

Thankfully I only have 4 websites to transfer (one of which is a client's).

But I'm wondering the best way to go about it. It appears that it isn't possible to transfer one account at a time?

Let's say my main domain is www.maindomain.com. If I repoint this to my new hosting account then won't my other 3 websites go offline when my domain propagates?

What I'd like to do, is transfer over my WHM settings, and my main website (is that what it's called?) and then transfer the 3 other websites at a later date, leaving them on my old hosting account for now.

But I imagine that if I update the nameservers for maindomain.com then any of the cPanel accounts that are registered within it will go offline unless they are transferred over to the new hosting?

Does this mean that I will have to move all 4 websites in one go, and then update the nameservers for each of the 4 domains at the same time?

That won't be too much hassle for me but what if I had, say, 300 cPanel accounts for example?!

You can actually transfer 1 account at a time with Cpanel, but for transferring all accounts at once, you can use the "Copy multiple accounts/packages from another server" menu.

I suggest using new nameservers for the sites you already transferred so the 3 remaining websites can stay online on the 'old' server. If you use the current nameservers and update IPs, the 3 sites hosted on the other server (old one) won't work anymore.

Let's say my main domain is www.maindomain.com. If I repoint this to my new hosting account then won't my other 3 websites go offline when my domain propagates?

Only if they are aliased to that hostname.

Thanks for the advice. I think I may just purchase a separate domain purely to act as the main domain on my reseller account.

You can move the files over using various techniques. WHM has some built in features, and it's probably best to use them. In terms of your nameservers, you can edit the DNS records on the server and point them to your new server, while also changing their records via your registrar to point to the new server.

This way even while the changes have not propogated, when the records are called on the local server, they'll automatically point them to the new server. You probably don't have access to your nameserver's DNS records on the server, so you'll need to contact your host for that.