and I need to convert them into old Locus tag which are
RS09560 NA
RS10020 1984
RS10595 2097
NA are the ones with not available old locus tag. I couldn't find any database to do so. And the information available for old locus tag and new locus tag is in the NCBI website
I tried to scarp the data from the website using R (rvest) and had written a few lines, but skeptical about the HTML node I should extract from the web page to get the information about old locus tag and new locus tag.
I wouldn't expect to find any source that has any old locus names that are missing from the NCBI genbank files. Gene annotations are constantly updated and it's not unusual for 'novel' genes to pop up, which will not have an old locus name.
Assuming that what you have is a neat list of accessions of interest, it is just a matter of looping through the 'Features' and looking for the accessions, grabbing the old locus names if available, and print a simple list of new vs old accessions.
Even if you have not touched python yet, it is a fairly easy and decent introduction to it if you are willing to give it a go.