I think replacing golang-github-golang-protobuf-1-3-dev with
golang-github-golang-protobuf-1-5-dev should be fine, as long as the
generated pb.go files are still produced by protoc-gen-go-1-3.
That's the easy part. In fact, (unless I'm overlooking something), we currently no longer have any package in sid that depends on golang-github-golang-protobuf-1-3-dev without an alternative on golang-github-golang-protobuf-1-5-dev. I think we are ready for the next step, which is rebuilding the archive with protoc-gen-go-1-5
Though removing src:golang-github-golang-protobuf-1-3 probably will not be easy.
Can you please elaborate on where you see the difficulties?