Conditional multi projects builds using sbt

Question

I have multiple projects set up in a repo. Example -

-root
    -project a
    -project b
    -project c
    -build.sbt

Currently I am publishing all the projects separately, and as an assembled jar (containing all the projects). Can I do something like a conditional build where I only publish a project jar if source code has changed? If it hasn't, sbt picks up the latest version from artifactory, and uses that in the assembled jar. If the code has changed, it proceeds normally by publishing the project as usual.