[support] upgrade from Drupal 4.7 to 5.3

[will hall]

Hi all, i need to upgrade a template for a client from 4.7 to 5.3.

Swapping old versions of modules with 5.3-compatible ones is pretty 
straightforward. however, I am not super-confident with php, and this 
page has me worried:

http://drupal.org/node/64292

Can I reasonably expect that if I go through all the template files in 
/themes/clienttemplate, and replace any of the functions the upgrade 
node says are new, that the upgrade will work?

what i want to avoid is getting in over my head, or causing the client's 
site to go down as I scramble to get someone to help me with an upgrade 
that I can't pull off. Also, i need to give the client a reasonable 
estimate of how much work the upgrade will take.

thanks!

will