Lol, I'd say it's medium size... I used an ecommerce wesite tutorial from youtube. There are about 10 pages that have mysql code in it. If tried to convert the code, the site would never go live.
You should really post this question on the Business Exchange Section. It's more appropriate because you are asking or paying someone to do some work.
The change from mysql to mysqli could be reasonably straightforward, depending on how the mysql is used in the code, e.g. classes/functions or vanilla embedding. PDO may be a little more work, but I'd go PDO as I find it cleaner.
Are you interested in doing this yourself if we're able to show you how? If yes, post snippets of code containing mysql usage. If not, as LM says, go to Business Exchange.