how to train on a new dataset after the engine had been created?

[AlyShmahell]

I have an engine I created with ./mmt create using an old dataset I had, and I would like to retrain it again using a new dataset for more fine-tuning.
I've tried ./mmt train --from-model but it messes up the BLEU Score.
I also tried creating a new engine from scratch with ./mmt create to train it on both datasets together, but I got an exit code -11 while training, i'm assuming the jvm ran out of memory at some point (it was using a lot while I was monitoring memory usage).
I've seen from other issues people suggesting ./mmt memory import -p parallel_src parallel_tgt -e model_name, but i'm not sure how much this impacts the performance or translation speed.
Any tips or suggestions are appreciated, thank you.