It depends on the counts but you might consider buying Drake PPR for 2019 just so you can do the organizers (1/3 of 30 returns, not worth it; 1/3 of 300 returns, probably worth it).  There's a minimum charge for the software but no extra charge to do an unlimited amount of organizers, the PPR only kicks in when you do that actual returns.  That's probably a bad short-term solution for a long-term problem.  Without converted prior data files, you're going to have to enter 1/3 of your clients from scratch into ProSeries for 2019 tax prep.  Start now.  Also, never assume that any conversion program is going to be 100% compatible.  I don't know about Drake > ProSeries specifically, but just because of how PS stores data, I would not expect any of your state data to transfer and I would look very carefully at federal data, particularly depreciation and any carryovers (such as suspended PALs, capital loss, overpayments applied, etc.)

Obviously it depends on the complexity of the returns involved, but the last time I changed software I opened the converted prior year return for every client in the new software and compared it to what was actually computed and filed in the old software before doing any future returns in the new software (I hope that made sense).  So, in your case, going from Drake 2018 > PS 2019, I would open every return in PS 2018 (including state returns) and compare it to what was filed with Drake in 2018.  Then you have a clean start for converting PS 2018 to PS 2019 and much fewer headaches during tax season.  After a few of them you get a feel for things that do not convert over well so you know where to start with the rest of the clients.  The first week of December might be a little late to start on such a project but every second/minute/hour/day you spend now is time you don't have to spend Jan.-Apr.  This is why most people are reluctant to change software (and rightfully so, there's a huge up-front cost in time and manpower to do the conversion correctly, and then a much higher risk that something will slip through the cracks in the middle of tax season and then you're stuck paying penalties with unhappy clients).

Rick