Is it permissible to dispose of a vowed sheep after it has aged and before the desired outcome has been achieved, and to fulfill its vow with another sheep of the same specifications later?

If the condition upon which the vow was made is not met, then slaughtering the sheep is not obligatory. If it was slaughtered before the condition was met, it is considered sufficient by the majority of scholars if the condition is met thereafter. If you fear its spoilage, you may dispose of it and then slaughter a replacement when the vow is fulfilled. However, if you do not fear its spoilage, you are obligated to keep it, as it is not permissible to replace it with another. If it perishes before the condition of the vow is met, no replacement is required.