As far as i know, if you move a property from aspect to type based property and remove aspect definition all together from content model, the properties on share ui won't show and fail to load the proprties due to Dictionary exception for existing nodes (if aspect was applied and property was already set). The error you are seeing is expected. For new nodes its not a problem anyway because they are created with new model change you made.
From user's point of view it doesn't matter where you keep a property. To keep the things simple i would suggest to keep the aspect as is and instead mark that aspect as a mandatory aspect on type.
If you were to remove the aspect definition for your specific use case, and if you have several nodes that holds the property and value from earlier defined aspect i.e. ab:xyzAspect and since property name is same that you are moving, you have to follow this type of approach consodering you want to preserve the already set property value.
1- Create a temp aspect with a temp property
2- Copy property value (value from ab:abc) from existing aspect (ab:xyzAspect) to temp aspect's property and apply the aspect on all the existing nodes.
3- Remove aspect e.g. ab:xyzAspect from all the existing nodes
Now your previous aspect's (ab:xyzAspect) property (ab:abc) value is preserved on a temp aspect.
4- Update the type definition with ab:abc as property, and remove ab:xyzAspect definition from content model.
5- Copy the earlier saved values from temp aspect's property to newly added property (type based) on all existing nodes.
6- Remove the temp aspect from all existing nodes.
7- Remove the temp aspect definition from content model.
Now ab:abc property on type will have the values that was originally saved.
(ACSCE, AWS SAA, Azure Admin)