TSEM9856 | Property held jointly by married couples or civil partners: Form 17 rule - further change in beneficial interests
From HM Revenue & Customs · Trusts, Settlements and Estates Manual
A couple might declare that their interest in property is split 60/40. Later their interests change so that they hold it 80/20. If they wish they may make a fresh declaration to reflect the new split. But it must reflect the actual position.