The term that you are looking for is fine-tuning, (or in this case perhaps further pretraining because it is the same task objective). You can easily do that with the example scripts that are available in the Github repository 45.
As @rgwatwormhill notes, you cannot create a new vocabulary and use that with a pretrained model: that model is trained with a specific vocabulary, and swapping that out for another vocab will lead to unexpected results. If I were you I’d first try fine-tuning the existing model with the script above, and if the performance is not good enough for you, you can still train from scratch. Note that you need a lot of data to get good results (that’s why typically finetuning is preferred).