Not that I'm disagreeing with the other replies, but I guess I'm looking at it from a different perspective. A lot depends on state real estate law, but a lot also depends on the contents of the recorded mortgage.
If you record a mortgage separate from the note, you probably also include a description of indebtedness. If there is language in the mortgage referring to any "extensions, modifications, etc), you may be covered by that, or you may want to include it in the indebtedness description you usually use. A real estate lawyer should be able to provide you sample language if it isn't in your standard form. If you have an open-end (future advance) clause in the recorded mortgage, that will usually cover what you are doing. In some states, lenders record deed/mortgage modifications.
I think you have to look at the original recorded document along with what you are doing, plus standard practices. I know it's not fun to read some of these forms, but that's where I'd start.