Unfortunately, I couldn't find the answer to my question concerning this subject and I've found a solution so wanted to share it with you.
When you're trying to convert a document into another class while also changing its workflow, you need to use Vault.ObjectPropertyOperations.SetAllProperties and setting AllowModifyingCheckedInObject to TRUE
Now, you could have a required property on the other class and you'd need to add to the current document. So this is how you could do it :
' Adding the mandatory date to the current document dim datePropVal : Set datePropVal = CreateObject("MFilesAPI.PropertyValue") datePropVal.PropertyDef = CONFIRMATION_DATE_PROP datePropVal.TypedValue.SetValue MFDatatypeDate, Date() Vault.ObjectPropertyOperations.SetProperty ObjVer, datePropVal 'Add the created property value with the date into the current PropertyValues Object PropertyValues.Add 0, datePropVal 'Change the class ID PropertyValues.SearchForProperty(CLASS_ID).TypedValue.SetValue MFDataTypeLookup, CONFIRMATION_CLASS_ID 'Set All properties to Allow modifying checked in object Vault.ObjectPropertyOperations.SetAllProperties ObjVer, true, PropertyValues
However, if you have any suggestions, tips for this particular case, do not hesitate :)