1. As a general rule liabilities are not assignable. 'This is common sense'.
Robson and Sharpe v. Drummond 1831

2. Exceptions:
a) If the parties agree that the duties of one of them shall be assignable.
b) Liabilities may be transferred by novation.
c) Liabilities may be vicariously performed by a third party on behalf of the person bound; unless the service is personal or it is agreed that a certain individual perform it. - the last two are not really exceptions -