Are ήρθε and ήλθε interchangeable? Is there a difference in meaning?

No difference in meaning.

ήλθε is the archaic form. ήρθε is the vernacular form, and represents a regular sound change in the modern language.

ήρθε is now the unmarked verb form. If you use ήλθε, you will come across as speaking in Puristic (Katharevousa); 100 years ago, that made you be educated, 50 years ago, that made you a conservative, and nowadays, it makes you an oddity.

There will be some verb compounds where the old ήλθε form makes more sense; certainly in the Katharevousa-derived verb κατήλθε στις εκλογές “he ran [“down-went”] for election”, the vernacular *κατήρθε would be absurd. On its own as a verb for “came”, though, it’s hard to justify in contemporary use.