Upgrade Live Store to v1.5.4 AND PHP 5.4 from v1.5.0 and PHP 1.2.17

I want to do 2 thing: (1) Upgrade my zen-cart from v1.5.0 to v1.5.4 and (2) Upgrade to either PHP 5.4 or PHP 5.4 (single php.ini) or PHP 5.4 (FastCGI).

However, I believe that upgrading to PHP 5.4 will break my live store as BlueHost informs me that zen-cart v1.5.0 is incompatible with PHP 5.4+
My live store is zen-cart v1.5.0 operating with PHP 5.2.17 in a shared hosting environment under a Linux Apache Server with a dedicated IP at BlueHost.

My prioirty is to minimize store down time. Since this type of problem is not unique, I'm hoping someone might have some suggestions on how to proceed and in what order. I have already done a successful practice install of the v1.5.4 with demo data per /docs/2.readme_how_to_upgrade.html and can follow those instructions.

I also want to make my store mobile friendly to meet google's new requirements, but thought it wise to take care of the above upgrades before installing any plugins.

Thank you in advance to any kind souls who are inclined to help!

Live Store:
Zen Cart v1.5.0
No plugins
Zen Cart installed via FTP and run zc_install
PHP Version: 5.2.17 (Zend: 2.2.0)
MySQL 5.5.42-37.1-log

Say have "practiced" the upgrade of the files. There are instructions included here about how the site can be operated using PHP 5.2.17: http://www.zen-cart.com/showthread.p...s)-with-v1-5-4

So you could do the initial upgrade having modified the 1.5.4 files as described following: http://www.zen-cart.com/entry.php?3-...d-of-upgrading

Then upgrade the php and restore the files modified above in the first step.

If the upgrade involves moving to a different server then some of the above would need to be modified accordingly.

Or... You can try patching the 1.5.0 to work with PHP 5.4 while you are upgrading. May be less work.
https://www.zen-cart.com/showthread....HP-5-3-5-4-5-5