My guess is this:
If they pick up 200/300s it is to replace the 100s. The ones PDT owns are tired and very maintenance intensive. They would replace the 37 seaters with true, unrestricted performance. The 300s would serve to increase seat capacity on routes that on the 100s run full and profitable without expanding staffing needs. And if they don't do a straight park a 100 for each 200/300, it will be a ratio. This would only happen if they find a home for the LGA aircraft (as in the number of aircraft, not the actual airframes). If there was no hope of placing the LGA aircraft, they'd just do a 1 for 1 swap, keep 44 a/c with 22 300s, 14(?) 200s, and the remainder as the newest 100s we already have. No net staffing change, just more capacity for the niche routes PDT actually is very productive on, 37 seat cap, not 32-35 ave for the lower capacity routes, and some old 100s just to screw with pilots in CQ (and any newhires in 2017) to have to know 3 sets of limitations (again).

My 2 cents.