Multiple settlors into a s89 VBT

I have been asked by a family who have set up a discretionary trust with the option to convert to a s89 in the future as the likely ‘primary beneficiary is vulnerable.

I know if we end up with grandparent and parents via their wills transferring money in to the trust each new settlement is treated a separate trust for IHT purpose, therefore under a discretionary trust a whole load of potential issues, what I cant see is if we convert to a s89 there would be any, I have looked at the HMRC notes which confirms the position for a discretionary trust but not s89.

Does anyone have any thoughts.