Can I test my already trained model using the data it was trained on?
It is possible to re-use the train+test dataset.

You used the train dataset to train your parameters and the test to check if there was or not overfitting in your model.

The train/test split is used to check your if your model is likely to suffer from overfitting or not, more than to obtain the "real parameters".

So, in that order of ideas, you discarded your model's overfitting and then you could use your complete dataset to recalibrate your parameters.

Remember the decision of wheter an individual belongs to train or test is arbitrary.