Contrary to what you seem to think, wouldn't and won't are almost never interchangeable.
The simple negative won't is used for future negative actions or for refusals.
- I won't go to the store tomorrow if it's raining. (Future negative.)
- I won't go to the dance with you. (Refusal.)
The negative wouldn't is used for counterfactual statements, and for future statements embedded in a past-tense narrative.
- I wouldn't shout if I were you. (Counterfactual)
- He said he wouldn't like it. (Future embedded in past narrative.)
In every case here, replacing won't with wouldn't results in something either ungrammatical, or it changes the meaning of the sentence.
Edit: An additional requirement for will/would is tense concord, which means that subordinate verbs in a complex or compound sentence must agree in tense with the main verbs. This applies to the two halves of an if/then construction, as well as to verbs in relative clauses. For this purpose, will is considered to be present tense, and would is past tense. So you see things like:
In this case, the distinction between will/would doesn't carry any semantic weight, but is required by English grammar. Swapping will and would in any of the preceding sentences results in an ungrammatical utterance.