So I changed the NS records with registrar to point at the new webhosts DNS servers and edited the SOA record there, deleting the new hosts default MX records and instead putting in the old ones for the old web\mail hosts. The website A record is however pointing at the new webhosts servers and the site comes up fine.

But none of this should cause me to loose access to mailboxes on my old hosts mail server right? I log into the control panel on the old host, all the mailboxes are there, all the passwords are fine but I can't log in using either webmail or pop3, says incorrect log-in/password. I even created a new mailbox and password for it respectively, but it would not let me log in.

For what its worth I did not change\delete the records for 'A' on the old webhost zone file, since I am not hosting the site with them anymore and NS records are pointing to other hosts DNS servers/zone file so that shouldn't matter right?

The old hosts mailserver is also not simply down, I can tell because through the control panel I setup a mail forward for one of the existing inboxes and when sending mail to it, it receives it and forwards it fine. So from this I can deduce that I have correctly inputted the old hosts MX records into the zone file hosted on the new hosts DNS and the mail is being sent to the old hosts mail server(s) and is successfully forwarded by it. But why can't I log into those account/inboxes anymore ?