P - paste the '- TopicEntry' line above our current entry
2x - delete the '-'
yyp - duplicate this TopicEntry line
Vr= - select this line and replace all the characters with =

So we'd get

TopicEntry
==========

(Which obviously looks better in monospaced font)

And now we can add bits to our #TopicJournal

This isn't quite perfect, as I discovered. If you sort to the last item in the table of contents then you can't +w*, and if another topic references the NextTopic, oops!