This utility will take OSGB36 National Grid coordinates derived from the old OSTN02/OSGM02 transformation and convert them to be compatible with coordinates derived from the current OSTN15/OSGM15 transformation.
The process is to transform back to ETRS89 using OSTN02/OSGM02 and then transform the ETRS89 coordinates forward to OSGB36 National Grid using OSTN15/OSGM15.
NOTE - this utility does not take into account the relatively small shift in OS Net coordinates from OS Net v2001 to OS Net-v2009. OS Net v2009 coordinates were released at the same time as OSTN15/OSGM15 (August 2016). For more information see this article and "A guide to coordinate systems in Great Britain" (particularly section 6).
The process is to transform back to ETRS89 using OSTN02/OSGM02 and then transform the ETRS89 coordinates forward to OSGB36 National Grid using OSTN15/OSGM15.
NOTE - this utility does not take into account the relatively small shift in OS Net coordinates from OS Net v2001 to OS Net-v2009. OS Net v2009 coordinates were released at the same time as OSTN15/OSGM15 (August 2016). For more information see this article and "A guide to coordinate systems in Great Britain" (particularly section 6).