I hope my comments will not be too akin to saying I would not start from where you apparently are!
I have encountered this issue in drafting definitions of specific gifts of shares in trusts and wills (and definitions in some commercial documents e.g. employee share schemes). The risk with a Will is ademption, potentially catastrophic. In other cases there is a risk, if a company’s capital is altered or reconstructed, of consequent ambiguity, but if a company is taken over by another cash debt or shares will replace the original shares.
Similarly a specific gift of an unincorporated business will be adeemed, if disposed of for shares before a codicil or new Will is made, in the absence of precise drafting.
The substitutionary provisions of IHT business and agricultural property reliefs are a guide to how a draftsman might approach the preventative methodology.
If you are stuck with a definition of “X Ltd or its successors in business” and X Ltd’s shares are replaced by holding company shares in my view Holdings is not a successor.
A successor would probably include a company to which the business of X Ltd was transferred in a company law reconstruction, by liquidation distribution or directly by court order, methods which can attract CGT reliefs for both share and business disposals. Arguably, faced with a fait accompli, a sale of its business by X Ltd to another company for shares issued to X Ltd itself would make that other company a “successor”; but I’d rather avoid that dilemma by drafting and the “other forms” you mention may well encompass that.
If I were entering into an agreement I would want to know that my shares could not be expropriated, or their value diluted, without my consent by accidental or deliberate actions of others. If I were an existing party to an agreement I would be hoping that the agreement or the company’s constitution allowed me (or the trustees if I were a beneficiary) to block such an occurrence. Minority protection in company law might be available but ideally is a last resort.
If the event has already occurred much will depend on the context in which the definition is operative; but the risk of ademption of a gift in a Will is as close to certain as it would definitely be if the shares had been disposed of for cash rather than in exchange for other shares.