You don’t have to edit in HD resolution.
You don’t have to edit in 23.98p either if this is what you used to shoot your material. Working in full HD resolution would unnecessarily burden your system and your storage. It is safe to work with SD downconversions in 29.97 as long as they are properly made.
Various online NLEs have different ways of accommodating the transfer of your project from SD to HD so check with your post production facility first to find out what they will need in order to finish your project.
In most cases it is the best to use offline and online systems by the same manufacturer i.e. Avid Media Composer for offline and Avid DS Nitris for online but it is not a hard rule.