Bert, I do that too, but usually unintentionally. I have an identifiable tune in my head in the beginning of the day, but later on it turns into something entirely different. If I can manage to write it down, I've got a new tune. I did an experiment once to see if I could do it on purpose. I started with Morrison's Jig and it ended up as this:

T:Frosty's Denial
M:6/8
L:1/8
Q:140
R:Jig
K:Em
||:EDE B2B| BcA BGD|EDE BAG|FAF DEF|
EDE B2B|BcA BGE|GFG ABc|BAF E2F:|
GBB dBB| eBB dBd| edB cBA| BAG ABd|
dBB gBB| fBB eBB|AFD DEF| GAF E2D:|
GEE BEE| cEE BAG|AFD DEF| GBA GFE|
GEE BEE| cEE BAG|AFD DcB| AFD E2F:||

Sometimes I've written tunes from scratch based on a feeling. I think about what melody would most express that feeling, usually a bar at a time. (aside to SueH - that's how Dragonfly happened) This is much more difficult than the "tune mutation process" since I do it conciously instead of just letting it happen.