Switching from KT120 to 6550, just plug&play or dial back bias?

Oldhvymec answered in a complete and accurate fashion. 

Biasing "low" means increasing or decreasing the negative bias voltage?

Audio Research VS115

yes, turn down the bias to about 50-60% of what it was driving the kt120’s, and then insert 6550/kt88, then turn on, wait 5-10 min, and bias up from there to the target 65 mv across the test resistor

https://forum.audiogon.com/discussions/audio-research-vs-115-bias-adjustments

a good reference post for you, op  ^^^^

Interesting outcome. It didn't change the overall neutral character going from KT120 (TungSol) to 6550 (Shunguang), still very happy with the entire setup. Immediate impression was...it's a bit on the lean side, bloodless and thin...but that changed from song to song, on others I indulged into the more precise, refined and thorough presentation. As time went on...especially around voices there was more detail. It eventually came down to two different flavors, right now I like the 6550 better and can't wait to find a detail here and there that I hadn't picked up through the KT120.
The setup is USB/streaming through a Raspberry Pi, then Digi +v1.0 into dbx venu 360 loudspeaker management system, splitting the signal at 50Hz to open baffle Beyma 15 B 100/R (Alesis RA150 driven) and Quad 63s (ARC VS115 driven).
Thanks again for the help,
brxl