Yes you are breaching their contracts (if a higher level has been agreed even verbally) and generally this needs to be agreed by both parties.

What most people would do is to standardize for new starters.

Is it you that want standardisation or is the Board asking you to look at this?

If you, then why? It is not HR's place to do this without being requested - just because you think everyone should be 'standard' does not necessarily equate to what senior managers want (or can get away with for their departments), whether this is holidays or any other areas (benefits, hours of work, ad infinitum).

Is it the Board? Then ask them why they want it done. What issues is this causing and, more specifically, in what areas (just differing holiday entitlements or are there other T&C's where there are variances? - each should be looked at individually and not altogether)? What do they hope to gain (apart from dissatisfaction from those who will lose out by changing their entitlements).

I have worked within several industries and diverse companies (size, location, etc) and have often had differences from 'standard' - mostly these were negotiated by the individual employee on recruitment or due to seniority or TUPE. I have also worked were different areas of the business have varying holiday entitlement and rules to others and I have never found it an issue (a little more work for hr maybe but that should not be the reason for standardisation).

By all means confirm individual holiday entitlements and other T&C's in writing (in fact they should be!!) but this does not necessarily mean everyone being on the same ones. With SME's I have often found that the flexibility in T&C;s that you can offer the right people when recruiting can be key. I.e. Have a 'standard' entitlement but be flexible to offer more if this will get you the person you really want/need.