Can I tune a model after training it? (Convolutional Neural Network & Classification)
Before answering your questions, let's understand a few points -
1. Model training is nothing but finding the right weights going through the guiding path based on the loss function o/p using y_true and y_pred.
Training starts with random initialization of weights(definitely following some rules)
2. Neural Network training is an incremental training approach i.e. it doesn't see all the data in one go like the DecisionTree

Can I use the current model with weights and bias to retrain on augmented data (from the same initial dataset)? I am guessing there are 2 possible outcomes, it tunes the model or it &quot;resets&quot; the weights and bias

Yes, learning will continue without any big disruption as augmented images will produce a similar gradient like the real images.
It will not reset, as I said the training is an incremental process.
Any new image which has completely different features can definitely cause a large gradient to flow and might disrupt the weights but with augmented images, I don't think any such thing should happen.

Can I perform hyperparameter tuning with RandomSearchCV or GridSearchCV using on my existing model? I believe this will tune the model.

You can do this but you don't have too many hyperparameters left as Model is fixed. You may tune Learning rate.