Convert a Polygon ( Close way ) to a node with its tags.

I do not understand why one would like to do such thing but there may be situations suggesting such a simplification.

That's not too difficult with JOSM:

• create an "empty" new node
• select the "old" way
• copy it with "Ctrl-c"
• select the new node
• paste the way's propertys into the node with "Shift-Ctrl-V"
• add a friendly note Tag to the new node containing "way/nnnnn" with nnnn being the id of the old way. doing this you keep a hint to the history of this object in OSM. I think that is important.
• now you may delete the "old" way. (perhaps also containing the id of it in the changeset comment)