Thank you very much @jetdv for the insight, that's very helpful - therefore presumably the most efficient option I have is running a script to create a new Titles and Text box wherever there are legacy text and then manually copy and pasting the text over?

Had another idea, perhaps I could export just the Legacy titles video tracks as an.mp4, then import it into the project - then I wonder if there was a way I could add a black outline around any text that appears using an effect/plugin..

perhaps I could export just the Legacy titles video tracks as an.mp4, then import it into the project

You'll then need to change the Compositing Mode in the track header to 'Add' or 'Screen' in order for just the titles to display over the lower video tracks (so that the black background behind the titling becomes invisible). It will be interesting to see if any FX can add a black outline (e.g. a drop shadow) around the titles because anything black becomes invisible via the compositing mode process. I tried adding BCC Drop Shadow FX on an .mp4 render of a title using the above approach and even using a color drop shadow does not work. I think that this is because the FX sees the .mp4 render as being a complete video (i.e. a full screen black image with lettering on it) and thus much different than occurs when using a titler. To test this, reducing the .mp4 size using pan/crop shows the drop shadow added to the edges of the video event:

Export your titles to a codec that supports an Alpha channel, then you can superimpose them over any video easily. Try using the track motion drop shadow or outline to create the effect you want.