I'm looking to upgrade to a Dillon 550.

Can I use my Lee dies or is the Dillon system a closed system only allowing their proprietary dies?

Thanks in advance

Standard Lee 7/8" dies work just fine in all Dillon presses except the SDB.  I used 6 different sets of Lee Deluxe Carbide die sets for my straight wall pistol calibers on my 550B and now use them in my XL650 with excellent results.

The only problems one may in encounter is the short body size of the some Lee dies may have you putting the die lock ring on the bottom of the tool head.

As previously mention the only proprietary dies are the Dillon Square Deal B Dies.

If buying new dies....buy either Dillon or Redding Pro Series dies as all other die sets you are paying for an expander die you will never use. And, you will end up buying a separate crimp die. I am a firm believer of seating is one step and crimping is another.

Many older sets of dies for autoloading handgun rounds came with ROLL crimp seater dies not a taper crimp seater die.  Hornady recently and RCBS recently with last few years have made changes in their offerings....

Just a matter of having the right tools for the round you are loading for.

FWIW... I always recommend Lee carbide sizing dies (but like the Dillon dies for the rest of the stations) for auto loading straight wall pistol cases, especially 9mm, as they size further down the case... which prevents chambering problems down the line..  if you get the 550, get the hard stand and roller handle... stand up... to help prevent tendonitis if you plan on loading for something like USPSA competition... add a light so you can see into the case at the bullet seating station and check for powder

I recently picked up a 550, and am using lee dies from my last press. The dillion lock rings are nice, inexpensive and smaller than the lee which gives you a bit more space.

I am currently using Lee dies in my 650. They will work in the 550 as well. I like the Lee FCD [Factory Crimp Die] so I'll usually buy a Lee die set with one included or purchase separately if using RCBS/Redding dies. I have only used Lee dies in my 650 so far. Lock ring needs to go under the die plate, not on top if using Lee dies.

any dies will work

I only own Lee dies for common calibers, I use them on a 550 and a 650 with no issues..

I prefer Lee's bullet seating and crimping dies for pistol calibers over Dillon's because Lee's are hand adjustable for bullet seating depth and amount of crimp whereas the Dillon's require a wrench which is just one more thing to pick up and use.  For rifle calibers, I prefer Dillon's resizer die over Lee's because of it's carbide expander ball vs Lee's steel cone shaped expander which squeeks and is harder to push through the case without inside neck lube (unnecessary with Dillon's die).  I still couple the Dillon with Lee's other rifle dies because, like their pistol versions, the bullet seater is hand adjustable vs the Dillon needing a wrench.