The features from the template are copied into the assembly, where they could be modied, reordered or deleted without dependency to the template. So there is no direct way to replace the "start assembly" within existing models.
Only for rule-based layers, you can define template models, which allow existing models to adapt their layers accordingly (config.pro option default_layer_model).
What can be done to adapt features or parameters from template models, is to apply the changes, that make the difference between both models.
As mentioned before in previous posts, ModelCheck or SmartUpdate can help automating this.
Gunter