In my game(s), you can adjust you L rating through roleplay. Their C rating stays as-is (unless the contact does something to raise it).
my personal opinion (and it is a jaded one), is that if your contacts are only a source for _______ (whatever that may be) then they are less likely to help you out in the future. If you do little things for them, maybe give them a bday present, or urban brawl tickets for a team they really like, or just little things like that... that means they are more likely to be a better contact or worse.
for example"
lets say I was a bartender at a bar and I had a side business "getting items". If I had two customers (Mr Blue and Mr Green) that were equal customers (same C/L ratings). Now Mr Blue remembered that I liked this odd kinda rare scotch.. and found a bottle and gave it to me.. then send me a "happy holidays' card at the winter holidays (christmas).. and did little things like that.. Now Mr Green who has spent on "work" items that same amount, but never comes around unless he was something.
Now lets say the local beat cops busts me for have some "party favors" and is willing to let me slide if I can give him a "bigger fish"... now I have two customers that I can give him. Mr Blue and Mr Green. One is just a customer and one has some "friend"-like tendencies. It would be a no brainier to give up Mr Green over Mr Blue.
then again that is my person 0.02

worth....