Generally, yes it would be wrong. Even though StackExchange allows/encourages edits to all questions and answers, the original author's name is still the primary attribution for a post. Substantial content edits or additions are essentially putting words in the author's mouth.
This comes up periodically when somebody answers with objectively incorrect information or statements. The voting system, not editing, is how these should get handled. Assuming it's an otherwise legitimate answer (meaning, it addresses the question), the poster has the right to be wrong, and voters have the right to penalize wrong answers. But it's still their answer.
In this case, as in addressing objectively wrong answers, the correct course would be to create a new answer at the old question, addressing film issues.
Additionally, the original older question might need to be edited slightly to make it more generic to both digital and film photography.