Upgrading from 1.3.6 to 1.3.9: Will db upgrade break existing 1.3.6 site?

I am upgrading a site from 1.3.6 to 1.39 installing to a new directory but keeping the existing database. I need to keep the existing site up and running until the customer signs off.

Can the upgrade from 1.3.6 to 1.3.9 break the existing site? Will the the database upgrades cause the existing site to fail?

Environment:
Database: MySQL 4.1.20
PHP Version: 4.4.9 (Zend: 1.3.0)
HTTP Server: Microsoft-IIS/6.0

Add ons:
Quick book import
ez populate

Update History:
Zen Cart 1.3.6
Database Patch Level: 1.3.6
v1.3.5 [2006-11-12 21:21:19] (Version Update 1.3.0.2->1.3.5)
v1.3.0.2 [2006-11-12 21:21:16] (Version Update 1.3.0.1->1.3.0.2)
v1.3.0.1 [2006-05-14 17:31:14] (Version Update 1.3.0->1.3.0.1)
v1.3.0 [2006-05-14 17:31:14] (Version Update 1.2.7->1.3.0)
v1.2.7 [2006-05-14 17:31:12] (Version Update 1.2.6->1.2.7)
v1.2.6 [2006-05-14 17:31:12] (Version Update 1.2.5->1.2.6)
v1.2.5 [2006-05-14 17:31:12] (Version Update 1.2.4->1.2.5)
v1.2.4 [2006-05-14 17:31:08] (Version Update 1.2.3->1.2.4)
v1.2.3 [2006-05-14 17:31:08] (Version Update 1.2.2->1.2.3)
v1.2.2 [2006-05-14 17:31:08] (Zen-Cart Core Files Version Info)


I would do my own testing on this issue but the test servers I have are PHP 5.2+ and break the 1.3.6 code.
thanks,
mackenzie

Nobody is going to be able to answer this question definitively. In theory it shouldn't. Zen Cart is designed to support these upgrades. But you've already raised a warning flag when you say that the existing site breaks when run under PHP 5.2.

So the only answer that we can really give is to do the upgrade in a test environment (which is what you should do anyway). The current version of Zen Cart should run fine on PHP 5.2, so you would see whether there is a problem baked into the old site that going to cause problems during upgrades, and deal with it before upgrading the live site.