Re BS Standards for Distribution boards and Enclosures

Distribution boards aren't cheap, nor are their sub circuit protection componants. We have old square D boards installed in the early 90s and we just obtain the Schneider equivalent if we need to add or alter a circuit to bring it in line with current standards.BS7671 will determine whether or not you need to change the likes of a mcb to a rcbo, or in the case of three phase circuits, just ad RCD protection via an external enclosure adjacent to the dist board.

Question 1 is rather vaugue - what are you defining as 'impact'? Number of spare ways left? Future capacity/loading?

Question 2 - the answer is the requirements of BS7671 which you should be familiar with if you are undertaking this kind of work.

Question 3 the answer is yes if available and the board/range isn't obsolete.

There are several questions here. I will kick off but others will no doubt spot things i have forgotten.

1)  Re 40 year old breakers to previous standards.  There is a broad overlap in the tripping speeds, but you may find in marginal cases, that the circuit is a bit frisker to disconnect with a new breaker than it once was, or that inrush that never matterd before now does.
how well do you know the history, in the sense of have they ever been called upon to trip, have there been any failures to trip, and is the environment cool, clean dry ? Contacts degrade with tripping, but in all devices lubricants in mechanisms slowly stiffen and if things never move, then sliding shiny parts slowly oxidise and become rough and may fail to operate when they should or perhaps even at all.  Damp induced corrosion I hope needs no explanation.
If the answer is anything other than these are perfect its probably time for a change.
2) RCDs. This is where the regs have changed over the years, and are now considerably more demanding. If in doubt look at your copy of the regs but basically, since 1989 we now have wiring routing rules, and wiring that is concealed and not armoured or in metal conduit needs to be RCD protected, all socket circuits for portable kit less than 63A needs RCD protection and circuits that may not always meet disconnection times due to length, may use an RCD to provide fault detection.  Note that there are  a few  of types of RCD, with varying performance for simple resistive and electronic faults, and the old AC only type is generally deprecated.

3) swaps of giblets. 
It is not guaranteed to fit - there is no standardization of contact positions and mounting offsets, even for older offerings from any one maker, let alone between different makers . If you do, you have to be happy that you have become the designer of the new enclosure and are adopting legal responsibility for its safe operation. Unless you are very confident, and ideally have the makers approval, the advice has to be no for anything more than an expedient fix to keep something running. As it may well involve moving DIN rails and bus bars, it may well not even be any cheaper. Of course if the maker is still in business and says its OK, or even makes an upgrade kit, just go for it.

regards Mike

I am not clear on what you are proposing to do.

If you mix and match, you become the manufacturer of the board and, accordingly, responsible for ensuring that it meets all the requirements of BS EN IEC 61439-3. That may be difficult if some of the parts are to BS 60439-3.

See Note 2 to 536.4.203.