Import comments from a model on Modelo so you can reuse all of them in the updated model.
When upload a model, check the option to "Import comments and animation from..." and select a model with 3D comments; the new model will have all the comments from previous model, in same position. This way, you will no longer need to worry about lose all your comments when you upload a updated model.
P.S. Only model uploaded after Oct.31.2016 is supported for this feature.