dbltoe:
Not quite as tough as putting a 6.7l Cummins diesel in a volkswagen BUT....
So much has been done since 1.5.5f (including some security updates) that it is actually safer to upgrade.
Put 1.5.7d in a new/test folder. Bring over images, define pages, and your template IF compatible. Put current versions of mods in the new site and import/update the old database.
All can be done while the old site is running.
Meanwhile, being in the UK, you do not mention which PayPal you are using and whether or not your host has changed PHP versions on you.
Do a google search for PayPal UK Zen Cart and look at some of the current threads on what's going on in your area.
Thanks for the detailed response, but I am a Zen Cart Developer and I realise all this. However I was asking a compatibility question. Some people are stubborn about upgrading, including this particular client as he was getting alot of business without upgrading.
Website Payments Pro is what is being used alongside Express Checkout. I have installed it on 1.5.5f anyway because he was getting 3D secure errors. The older version is backed up, and both have kept the same settings.
This may come across as the wrong way of doing things, but if a client won't upgrade to the latest zen cart, they won't, and there is nothing I can do about that until they accept their cart has fallen over. Well apart from try and keep them running on their current set up. They are running PHP 5.6 but obviously according to the server requirements I can take it up to PHP 7.1 if necessary.
Nick Smith - Venture Design and Print
https://venturedesignandprint.co.uk