[abhaskare]

I am thinking about moving on from my K3 and going full frame with the K1. Which of my lenses would I no longer be able to use if I decide on the full frame?

55-300
18-135
18-55
50-200
50 f1.8
35 f2.4
Sigma 10-20
Sigma18-35 f1.8

All the lenses you have are for apsc format. If you are thinking of K1 and spending 2K on the body, you would best benefit from getting the FF lenses. Personally I would like the 28-105 as it covers a wide range. The K1 has pretty good high ISO perf and IBIS so that f/2.8 might not give any advantage other then subject isolation/bokeh.

If you are thinking of reusing above lenses in crop mode on K1, then why bother at all, you already have a K3 !