+1...Completely ferment your beer, before lagering.
I love using corny kegs for lagering, because the less I have to transfer, the less there is to clean.
Some folks like to cold crash, prior to lagering...This is up to the brewer...No ill effects, either way.
Your yeast activity at the time of lager should basically be non-existant (I should say there is little/no co2 being formed...So, purging is not necessary).
The only reason I ever purge a keg is to release the air that I put in there, before I turned on the co2.
The phase of fermentation that produces co2 should be over, by the time you are ready to lager.
The only thing the yeast MIGHT be doing is cleaning up a few of their byproducts...
But, the cold temperature used for lagering should force them into a dormant state.