For me the main issue was motor interference because I used to play UWPS using wrist deviation and a good amount of thumb muting (thumb resting on lower strings) so my thumb was also my anchoring point. When I started learning DWPS when trying to built speed I was always falling back to UWPS I think due to the wrist deviation movement so I decided to built DWPS with arm rotation. My breakthrough happened after watching Teemu’s interview picking Rhoads and pentatonic examples. Now that I feel comfortable with DWPS to some extend, a new challenge has arisen since I use different motions for DWPS and UWPS but my end goal is to perfect 2WPS. Switching hand positions seems not to be the main issue but switching between arm/wrist movements so I will start experimenting with DWPS wrist deviation and UWPS arm rotation (opposite to what I do now, however wrist deviation DWPS feels easier than arm rotation UWPS).
Another part of my breakthrough was practicing without even thinking in my fretting hand. Honestly I not even had a guitar because I was traveling and the only thing I had was a Shred Neck (cut guitar arm gadget). I picked only between 2 strings (2 note per string pattern) for many hours. I used initially some sort of burst approach 4 pick motions and break, 4 pick motions and break (4 pick motions = 2 per string). Then I added an additional movement (not pick motion) which was get to next string to get ready for the next pick motion (that was my break for this new pattern). And I also separated higher to lower and lower to higher string changes. One thing I did not do was practicing super slow, do not get me wrong I did practice slow but not like quarter notes at 40 bpm. Honestly the burst practice seemed to work the best. And from that I built to more picking motion patterns between two adjacent string (8 motions for example 2 per string or 4 per string). One day I just picked up my metronome app and I was able to play sixteenth notes at 120 almost effortless between to adjacent strings 2 nite per string (I said notes here but it was just a series of pick movements such as having all strings muted with the left hand and just execute the right hand movements - still on the shred neck) and eventually I built up speed up to 150 bpm but only for short sequences.
I am not saying this will work for everybody but in my case this is the way I was able to break a solid 15 years UWPS mechanics in a 30 day period.
Off course the next step was translating all that shred neck practice to the actual guitar. At the beginning I thought I had lost my time and effort but after a couple days I started getting use to the guitar string tension (main issue was that the shred neck picking space is too close to the end so string tension is quite high and I was using to much strength for the actual requirements for the guitar).
One thing I still struggle with DWPS is playing in the 1st string because I got used to use the next higher string when playing in any given string as the reference point to stop (when I do a downstroke it ends with the pick touching the next higher string) and since there is no higher string when playing in the 1st I have no reference point to stop and loose some control. Currently I do not use my other right hand fingers for anchoring as Troy does for example so I am also evaluating if that could help me to improve this.