The Final 1.5.5?
1.5.5f for Zen Cart was released 31 Dec 2017 but contained only a few bug fixes and one major addition of Square now being a built-in payment module. That's great news for sure; however, Square should not be a major factor in deciding whether to upgrade now or not. The good news is that 1.5.5f is supposed to be the final stable version. That's why I've started upgrading.
If your host changes the server PHP version to 7, well, then upgrading becomes essential. I really hadn't worried so much about that until I realized that the hosting company that leases me my server is starting those upgrades. What I did see at a minimum, PayPal quits working. I can't tell you that pulling the files from 1.5.5 for PayPal and inserting them into your 1.5.4 cart will correct this. It might but no telling what other problems you will be left with.
Not all hosts mandate the PHP version on their servers. I do and most small hosts will, but a number of larger hosts will allow you to change the version to your choice so you may be quite a ways from facing this scenario. If you aren't sure, log into your cPanel account and check the php button. It will either state what version you have or there might various versions listed which means you get a choice.
Problems with Upgrading to 1.5.5
There are two issues one has to consider. 1st is about the mods. There are still mods running around that have not yet been updated to 1.5.5. And just because the plugins section at zencart.com does not list a version for 1.5.5 does not mean that it doesn't exist or that the 1.5.4 version won't work. This is a tread carefully situation and a very good reason to use a Zen Cart professional to do the upgrade.
Secondly, the other problem can be the PCI scans. If you have to have your site scanned for PCI compliance, upgrading to 1.5.5 without the PHP version being 7, can cause your site show as non-compliant because 1.5.5 is only PCI compliant on that PHP version. Will it fail? I cannot promise you that it will because different scanning companies do different things (my opinion – no proof here). So maybe best to upgrade close to the PHP version being upgraded as well. That means that you have to rely on being informed when that will be – a dicey decision! (Oh, by the way, if you are required to have PCI scans and aren't using Geekhost.ca for your hosting, you are missing out on a good thing. Contact them for more information.)
Contact WizTech for Upgrades
Pricing for upgrades vary depending on your present version, installed mods/plugins and several other factors. If I did your present zen cart installation, I am willing to skip doing a new site evaluation and might, just might, give you a break on the regular pricing. ($350 to $900)
Otherwise, I do require a site evaluation (presently $175) to prep for the upgrade. After the evaluation, I will give you a fixed price for your upgrade.
More about site evaluations:
http://www.wiztech4zc.com/zen-
To register/pay for a site evaluation:
https://www.wiztech4zc.com/
All Zen Cart upgrades are guaranteed. You pay a flat price and you get a perfect upgrade!
Right now I'm also offering something not normally covered in an upgrade. If you have a secure certificate, I will force HTTPS for your whole site and force either non www or www as well. Both are Google recommended SEO steps so just a bit of optimizing to accompany your Zen Cart upgrade!