I have some collections in Lightroom that references photos on my hard drive. If I remove those folders and reimport them, will the collections be broken?
If you remove the folders outside of Lightroom, you will not affect your collections, however they will report that the images are missing. If you recreate the folders outside of Lightroom in the same location with the same folder structure and file names, You won't need to reimport them, as Lightroom will automatically recognise when files in its catalog are present or not.
Lightroom stores any adjustments in its catalog, so it will remember them even when images are not present on the hard drive.
This is similar to working with offline files on an external or network drive.
If you re-import the images, you risk overwriting any adjustments in the Lightroom catalog.
If you remove the folders from within Lightroom, you will also remove any images in them from any collections they are in.
That depends on what you mean by "remove those folders". If you simply remove the folders from your hard disk and then re-import them into Lightroom (from a different location I presume) the library might not like that (or it would just ignore them as suspected duplicates).
If you remove the folders from your Lightroom library and then re-import them, nothing bad should happen.
To be on the safe side, you could simply create a new temp library and try it there with only a few photos.