There are many ways of upgrading existing doors that will achieve an improved fire safety performance though the results can never be guaranteed unless an identical door has been upgraded in an identical way and subjected to a full fire test.
But upgrades are often the only acceptable solution in a heritage building. It needs to be considered in conjunction with a fire risk assessment to determine the level of protection required from the doorsets, the standards of upgrade which is possible in the circumstances and whether any alternative methods of protection may be appropriate instead of or as well as the upgrades to the doors.
Do a google search and take a look at the English Heritage website, The ASDMA(Architectural and specialsit door manufacturers association), TRADA (fire doorsets by upgrading document) and some of the upgrade product manucterers such as sealmaster and envirograph. There are many more.