An accord and satisfactions is essentially a separate contract from the prior contract.  Typically, some lessor performance has been agreed to by the parties.  That part is know as the accord.  Execution of the "new" contract terms is deemed satisfaciton of the accord.  I saw one essay on this issue and of course comes up in the MBE's. 

Modification of contract concerns adding some new terms, changing terms, etc.  As you very well know, under the CL you need new consideration to support the modification while under the UCC, good faith is the requirment.

Modifications are changes to the same contract while accord and satisfaction is a new separate contract.  Hope this helps.  I'm sure someone will add to this but its how I see it. 

• mutual rescission if there is performance still remaining from each party - "let's call it off" UCC requires writing (UCC §2-209); (not after K performed, house already painted)

• subsequent modification;
• novation: mutual agreement to substitute new party in K; (,novation agreement is of both parties and original party is excused, in delegation promisee can sue delegator and delagatee.
• UCC good faith modification;
• substitute K that discharges old K;
• accord & satisfaction or account stated "alright, let's settle this";

Accord and Satisfaction (substituted performance)

You need to know (i) what an accord is and what a satisfaction is and (ii) the effect of the making of the accord, and (iii) the effect of getting no satisfaction.

1. MEANING OF "ACCORD", "SATISFACTION", accord question will have an agreement by the parties to an already existing obligation to accept a different performance in satisfaction of the existing obligation satisfaction that different performance. D borrows $1,000 from C and agrees to pay the loan with interest. Later D and C agree that IF D delivers 20 widgets by the end of the month, THEN the debt will be excused. Is this an accord? Yes

2. Effect of accord AND SATISFACTION

If the new agreement ('the accord) is performed (satisfaction), then performance of the original obligation is excused.

D borrows $1,000 from C and agrees to pay the loan with interest. Later D and C agree that IF D delivers 20 widgets by the end of the month, THEN the debt will be excused, D delivers the 20 widgets before the end of the month. $1,000 debt? What result? Excused b/c both accord and satisfaction.

If the accord is not performed, then the other party can sue on either the original obligation or the accord but not both: D borrows 1000 from C and agrees to pay the loan with interest. Later, D and C agree that if D delivers goods at the end of the month than his debt would be excused. D doesn't deliver. Yes, C can recover on either, but not both. Compare:

D borrows $1000 from C and agrees to repay the loan with interest. Later D and C agree that he D will deliver 20 widgets at the end of the month instead of paying 1000 with interest. D doesn't deliver. Can C sue for original loan agreement? No, it is excused by substituted agreement

• unilateral discharge by obligee - "you don't have to do it"

IMPRACTICALITY: Performance excused if more expensive/difficult to render performance then when K was made. Requires unforeseeable circumstance; impractical w/ respect to original K consideration, obligor did not assume risk. UCC - liberal, no foreseeability required.

condition subsequent: Occurrence releases D.