The English Wikipedia has a couple thousand WikiProjects, many of which are inactive, defunct, test pages, or too small to be viable. This page outlines a simple process for merging WikiProjects.
Post notices on the talk pages of the parent project and the project you are proposing to convert. Please don't surprise another group of editors by moving their pages without any notice. Keep the discussion in one of the two talk pages, with one of the notices being a link to the discussion on the other. Allow ample time for participants in a less-active group to object.
Generally speaking, if the merge seems appropriate, then you can proceed if either of the following is true:
There is active agreement from the affected groups.
There has been no response from the affected group(s), and it has been more than a month since you proposed the merge.
For example, if you want to merge WikiProject Tulips and WikiProject Lilies into WikiProject Plants, then you can proceed if:
Everyone agrees, or
Nobody objects after a month.
If one group supports (or doesn't object) to the merger but another does not, then proceed only with those groups that have not objected. If the 'target' (the intended bigger group) objects, then the proposed merge cannot happen.
Check for subpages and archive/move/redirect them as needed. Use Special:PrefixIndex to check for pages. Use Special:WhatLinksHere on each subpage to find any links that need to be updated. Most subpages can simply be redirected (e.g., WikiProject Tulips/Assessment can be redirected to Wikipedia:WikiProject Plants/Assessment). Some, such as lists of participants, may need to be merged.
Merge/redirect the banner templates before you try to deal with the categories.
Find all the categories that the merged-away groups have used. Generally, these are named something like Category:WikiProject Tulips or Category:WikiProject Tulips articles.
Any categories from the converted project which are empty should now be eligible for speedy deletion under either the empty category (C1) criteria for speedy deletion. Edit the category pages and add {{db-catempty}} to the top.
To do: Verifying that all the categories are working
The following discussion has been closed. Please do not modify it.
Check the categories on all the pages you moved or redirected, to make sure that they are not using a now-obsolete category (e.g., Category:WikiProject Tulips pages.
Make sure that automated quality assessment statistics are being generated correctly. If |TF_n_ASSESSMENT_CAT = Tulips articles is set for the task force, then categories of the type Category:FA-Class Tulips articles will be generated. Bring all these categories into a parent category Category:Tulips articles by quality, and categorise it under Category:Wikipedia 1.0 assessments. Assessments will be logged at Wikipedia:Version 1.0 Editorial Team/Tulips articles by quality log and overall statistics at User:WP 1.0 bot/Tables/Project/Tulips, which is transcluded on Wikipedia:Version 1.0 Editorial Team/Tulips articles by quality statistics. See Wikipedia:Version 1.0 Editorial Team/Using the bot for more information.
Update other project templates. Redirect, merge, or update existing userboxes, welcome and invitation message templates, and other project related templates.
A bot should handle double redirects, but after a day or so, you may want to make sure that obvious links, such as WP:WPTULIPS, are pointing at the correct page.