What a load of questions in one shotgun blast. Will try to answer in blocks of similar topics if I can.
Old Loco Conversions.
Within reason, any loco can be converted to DCC. But the older the loco is, the more potential issues will arise.
For one thing, older locos tend to have motors that draw far more current than more modern locos, particularly if they have worn weak magnets. It is not unheard of for some motors with weak magnets to draw up to 1 amp of current.
This raises the first issue of conversion. Hornby R8249 and TTS Sound decoders are limited to 500mA [0.5 amp] supported current.
Thus older locos can burn out Hornby decoders including TTS sound ones.
Any DCC conversion should be preceded by performing a 'Stall Test'. A stall test will identify the maximum current that the loco will draw and therefore the decoder current specification that needs to be sourced. If over 500mA, then a wider net needs to be cast outside the Hornby portfolio and this could preclude TTS as a viable option to consider.
This YouTube video elegantly shows the effect of weak motor magnets on motor current draw.
The second issue of conversion. Loco chassis and motor suitability. In order to install a DCC decoder it is necessary to electrically isolate both sides of the motor from the wheels and pickups. Some older locos use the metal chassis of the loco for one side of the electrical path. Thus making electrical isolation a little more difficult. Some much older locos use a 'split chassis' where both sides of the chassis [insulated from each other] provide the electrical path to each side of the motor. Thus older locos can really throw the spanner in the works when it comes to conversion methodologies. Then there is motor type. Hornby / Triang X03/04 motors are easy to convert, but Hornby / Triang Ringfield motors come in different variants and each have their own conversion quirks to overcome. This web page gives guidance on Ringfield conversion, scroll down the page that opens to the 'Ringfield' section.
Tender Driven Locos.
Tender driven locos tend to typically have one side of the tender motor connected to the tender chassis to pick up power from one side of the rails. The other side of the rails gets the power picked up from the wheels of the loco itself on the opposite loco side to the tender and passed back to the tender via an electrical coupling. These have their own conversion issues too, due to the potential difficulty isolating the motor.
Elite and Elite firmware.
There is no Version 3 or 4.5 firmware. I think you probably mean version 1.3 and version 1.45 firmware.
Firstly you need a Windows PC to update the firmware yourself. The firmware updater is not supported on an Apple IOS [Macintosh] platform. You first connect the Elite to the PC and let Windows [version 7, 8 or 10] load the driver, once the driver is successfully installed and the Elite recognised in Windows 'Device Manager' then you can start the firmware update process. All explained in the link below:
Updating Elite firmware [written for Ver1.43 but still the same for current version]
Alternative Elite firmware update method if documented method fails [see reply by Peter Rowney – 2nd post up from the bottom of the page].
Most users update the firmware without issue. It is usually only the 'users' for which it does not go smoothly who raise posts about it in this forum. I suggest, that it is more likely to go smoothly if following the documentation in the links above rather than Hornby's own 'How To' documentation, which quite frankly is poorly written.
TIP: As a newbie poster on the forum, just be aware that the 'Blue Button with the White Arrow' is not a 'Reply to this post' button. If you want to reply to any of the posts, scroll down and write your reply in the reply text box at the bottom of the page and click the Green 'Reply' button.
See also – further TIPs on how to get the best user experience from this forum.