Changing from static html to dynamic php

[milady]

Hi to everybody!

I'm working on my website since several years ago always with html codes. It has already a good position in Google. Now I need to change to php because I have to add an advance search.
I suppose (somebody else will do it because I have no idea about php) that the URLs are going to change, so it will be like a new start for Google.

How can I connect the old ones with the new ones (same domain) in order to don't loose the position I got after so many years?

Any help will be much appreciated!

Thanks in advance!

This is the website:
Apartments Buenos Aires. Buenos Aires Apartments