Is there a way to change fixed width Flat file connection manager to delimited without losing all the column names ?

Unfortunately not as it is a quite dramatic change causing the metadata being refreshed. I would try to open though the package file in a text editor and find them before doing the change, then after you made the change to the FF connector try to reconstitute / replace the autimatically generated columns with the old names.

I don't think so. Flat file conn manager will refresh the metadata. and would accept the  source colummns remapping. Unfortunately, SSIS is static metadata tool