Let me possibly put it another way...

Your site currently uses PHP 5.2.17 and ZC 1.5.0 supports that. You want to upgrade the site to ZC 1.5.4, but out of the box, it does not support PHP 5.2.17. The link provided (twice now) provdes guidance on how ZC 1.5.4 can be modified to work with PHP 5.2.17. There is a strong suggestion to upgrade PHP though instead of make the modifications or perhaps that is how it was understood.

The development team has taken great care to make t possible to upgrade from one version to the next or in some cases provided a documented path to do so. (Used to be that to go from pre 1.3.9h to 1.5.x had to upgrade at least twice before more sane instructions were provided. :) )

It is possible to put the new version on your. Current server without disrupting current service while incorporatng the changes necessary to do the "upgrade"... Sure not usng the "latest" PHP, but that will be the case at some point before the upgrade takes place...

I consider this me taking you to the water, if you drink, great, if not, well I think I've done what I can from here. A path has been provided, maybe it seems too easy. Yes there will be minor hiccups in the process, but what software doesn't have some sort of issue at some point on some server run by some "unknown" person(s)?