Please look at http://na.tm.agilent.com/8753/firmware/firmware.htm 
for version history and specific upgrade requirements.  
In some cases hardware upgrades are needed for some firmware updates 
but in most cases only the 4 EPROM's need to be replaced for rev 8753A-D 
and firmware floppy disk needs to be loaded for 8753E,ES,ET.  
Cal data is stored separately in an EEPROM.  I don't think instrument 
cal data is generally impacted  but I am not sure that is true in all cases. 
If you have access to a Floppy drive I think that you can save the 
instrument cal data.  The individual instrument states and sweep cal 
files may be impacted by the upgrade.  -Eric Haskell KC4YOE