Teams are only allowed to renegotiate a crew contract when that crew member has come to the end of his current contract or the team has moved up to a new division.
Additionally, the renegotiation period is during the new team sign ups (before recruiting begins but after teams have chosen to move up in divisions).
To reach the crew renegotiation page, click the Management option under the Crew menu. Once on the Crew Management page, click the name of the crew member you wish to renegotiate with to bring up their profile. If you are able to renegotiate with the crew member, those options will be shown under the Contract tab of the profile.