If I turn on iCloud photos on my iPad, will I lose old photos previously installed on my iPad from a PC via iTunes?

Yes you will. See the following from Get help with iCloud Photos - Apple Support

Any photos or videos you copy to your device through iTunes are not copied in iCloud Photos. To add your original, full resolution versions, follow these steps:

In addition to not being copied in iCloud Photos they will be removed from the iPad.

You can manually (by using an internet browser) upload those photos from your PC to iCloud.com Photos app and they will then sync back to your iPad or you can use the following instructions from Get help with iCloud Photos - Apple Support

If you're using a PC, learn how to upload photos and videos to iCloud Photos.