Moremi Game reserve (Botswana) geometry

@Paulo,

If the old relation is incorrect and the new one is correct and current, then simply delete the old one.

In JOSM, click on the old relation in order to edit it, then notice a trashcan icon at the right end of the toolbar at the top of the Relation dialog box. Click on it to delete the relation. You must also delete any individual ways that comprised the old relation unless they're also being used in the new one. In ID there must also be a way to delete a relation but I'm not familiar with that editor.

Also, you spelled Moremi wrong in the new relation: Moremie

The Moremi Game Reserve (3638339) has 13 members. You can expand the '13 members' text to see individual segments. https://www.openstreetmap.org/relation/3638339
If you want preserve history using Josm you could download the utilsplugin2 and use 'replace geometry' https://wiki.openstreetmap.org/wiki/JOSM/Plugins/utilsplugin2

I suggest you download the Moremi Game Reserve from osm into Josm but I expect it is much too big, so decide on a portion at a time to do.
Your new reserve shapefile? should be in a new layer .
I think you would have it in CRS: EPSG:4326 - WGS 84 for import into Josm. All the tags on the ways in the shapefile should be deleted too as you will use osm tags where required.

You could split your new shapefile into parts similar to the current relation and then select one of the segments and 'edit, merge selection' of that section into the osm data layer where the osm data is.
Then change/activate to the osm data layer, select a section of the current Moremi and the new line you just merged, then select 'more tools, replace geometry' then do the rest of the segments.
I notice that two segments of Moremi share segments with other lines/polygons/relations on the map
Way Kwhai river (272838429) as outer
Way Moremi Game Reserve (272837121) as outer
You need to decide to replace the Kwhai river segment with your line if more accurate or leave separate. If you decide to leave the Kwhai river segment as is and have your new line as a separate line, you will need to do that in the Relation Editor as mentioned by @AlaskaDave.
i.e. you would delete the Kwhai river segment from the relation and add your new line to it.
The current Way Moremi Game Reserve (272837121) should not have a tag on it as it's tags should only go on the relation.
There are several other segments that use river lines you will need to decide about.

When done you should have your new 13 segments forming a multipolygon and because you replaced the geometry, the Moremi Game Reserve should display your ways in the Relation editor. All you need to do is revise the tags on the relation.

You should run the Josm Validator to check for errors. You can select the Moremi Game Reserve and run the validation just on the newly created Moremi Game Reserve.
When you upload you might find the validator finds a few other bits to correct too.

You should list the source you used for the new data on upload.

This is a big job and I feel too complex if you are new to Josm so I suggest find a small multipolygon to try the process out on first and when you are more confident then do the Moremi Game Reserve.
If all goes badly, you can revert the whole/part of a changeset and start again.