upgrading my site.

[stitchnkitty]

I have been working on the version of zencart 1.55 (I think) php 5.6 www.stitchnframeonline.com
Customization to my site
the only module is standard shipping that is included and
the Ceon offline credit card module.

No other fancy modules installed.

I have not upgraded due to the constant purchasing from the overseas customers that purchase large quantities of my vintage kits.
The refund of large monetary amounts when this occurs is quite expensive.
My acct with my hosting company renewed and I was shifted to php 7.4 and of course nothing will work.
I am attempting to contact a webmaster but I assumed that he would need to at least SEE the site.
My thought was to upgrade to the lowest version that would "like" the php version of 7.4

What will the problem I can run into making this kind of jump?

Thoughts please?

[lat9]

I have been working on the version of zencart 1.55 (I think) php 5.6 www.stitchnframeonline.com
Customization to my site
the only module is standard shipping that is included and
the Ceon offline credit card module.

No other fancy modules installed.

I have not upgraded due to the constant purchasing from the overseas customers that purchase large quantities of my vintage kits.
The refund of large monetary amounts when this occurs is quite expensive.
My acct with my hosting company renewed and I was shifted to php 7.4 and of course nothing will work.
I am attempting to contact a webmaster but I assumed that he would need to at least SEE the site.
My thought was to upgrade to the lowest version that would "like" the php version of 7.4

What will the problem I can run into making this kind of jump?

Thoughts please?

Your best bet is to upgrade fully to the currently-released version (zc210). That has a bunch of in-core bugfixes and will keep your site going on now-current PHP versions as well.