I am thinking of swapping my shimano tiagra triple chainset for a double. will my STI shift levers for the front mech still be compatible, or will I also need to buy the double specific lever?
You shouldn't need to change your shifters if I remember rightly as you just don't use the 3rd gear on the shifter. It will take some fettling as you'll more than likely need to reset your derailleur height and limit screws but as the gap between the cogs is the same on a double than it is on a triple so shouldn't be any need to switch shifters at all..
Why do you want to change it?
I've started racing this year and want a bigger front ring
Seriously, if you can't keep up with 50 teeth on the front you won't manage it with 52. Or you could get an 11-tooth small cog on the cassette.
Change the cassette!
My Tiagra shifters form around 3 years back could handle a double or triple but it felt like a hack using them as a double. The shift indicator was out of sync as there are 3 spaces and only two gears and there was a spare shift down that did nothing. Don't get me wrong, it worked, it just felt like a bodge job.