Yes, you can roll back - or back out or reverse - the changes by reversing the two steps detailed in Migrating Your Clients' Trunks, that is:


(a) modify the PBX trunk settings, changing 'Outbound proxy hostname or IP'  back to be sip.500.uk.com (or alternatively back to the previous setting, which may instead be the 'SIP Domain' that you have set on your reseller 'Options' page on the SIP Trunk Plus portal)


(b) use the portal's 'Migration' tab to 'Repoint' the trunk's numbers back to sip-peering.500.uk.com


Follow the steps detailed in Migrating Your Clients' Trunks, using the values detailed in (a) and (b) above.