Can I set up a formula field so that a user can overwrite the information when in edit mode? Another approach would be to set the default value for one field to equal the value of another field. Is that possible?
Yes. A good use case for this, and the example I'm going to use, is when you have an Item price, but want to have the option to override the list price.
[My List Price] is a field coming from a parent table, or an items table. [Price Override] is a field that can change the value [Price] will be the formula field that takes the list price, or take the override price.
[Price] formula would be:
If( IsNull( [Price Override] ), [My List Price], [Price Override])
If you are using text fields, the similar, but use the sign ="" rather than the null: