I'm very new to drones. 
I have 2 beginner drones. Holy stone and wltoys.
I want to start upgrading into the higher end drones.
My question is, can I link up a transmitter like a Taranis X9D to my cheaper drones.

I saw this the other day.

Multiprotocol TX Module For Frsky X9D X9D Plus X12S Flysky TH9X 9XR PRO Transmitter
https://banggood.app.link/tNp8Ago0iG

It allows for binding to "Cheerson,Syma,Wltoys , Eachine E010(Most of these brand's quadcopters, helicopters and airplanes are compatible but not every model.Specific models please check instruction under the note)"

I wouldn't worry too much, as soon as you build your first "hobby grade" quad, your toy grades will just gather dust

I bought one of the multiprotocol TX modules that @derange suggested when I bought the components for my first build... just as @RENOV8R suggested, I haven't even taken it out of the packaging, once you have a serious quad you don't use the toy grades anymore, I've started handing them off to friends to try and get them interested in the hobby.