Replacing Stock 6x9s with Upgraded 6.5s and Rest of System

My car was totaled and I removed my stereo system to install into the new car. The set up is 4 6.5's, dual 12" sub, 5 channel amp, double din reciever. The car I want to buy is a Toyota Camry with stock setup of 4 6x9's and 2 tweeters. I have read that it is possible to put a 6.5 speaker in to replace the 6x9, with special mounting and adapter. I will have the installation done professionally, but they are closed due to 'stay-at--home" order, and i would like to get some answers now rather than wait (to help my car purchase decision). Do professional installers do modifications like this (like CarToys)? How many other peices of the system does it effect? For example, does this require special wiring, or is there a component I'm unaware of. How much extra is this cost besides what it would already cost to install?

I don't have tweeters, so I have to decide to upgrade (more money) or keep stock. This sounded great in my Honda as is.

We often go from a 6 x 9 to a 6.5, the adapter from metra work great. Unless you are going with some big power in the doors most of us will use the factory wire to supply sound to the door speakers.