Prepare your figures for optimal use of the medium at hand.
The main goal for your presentation will be to present the content and impact of your research and to advertise your paper to the audience. In my experience, the format of a journal paper figure is usually not the best to convey the most information in a slide presentation, and you also seem to be aware of this.
Since you want to make the presentation as convincing as possible, I would suggest to prepare any figure for the best possible appearance in the presentation, just as you had prepared the original figures for their best possible appearance in a paper. If a paper figure would waste beamer screen space, rescale it for the presentation. If color would help explaning the figure but the journal only has grayscale, add color for the presentation. And so on..
You want to win readers for your paper, and I am sure that most of them will recognize the figures and their meaning later when reading the paper. Those modifications may change the style or layout of the figure, but even then, the content of the figure (e.g. the general appearance or tendency of a graph or diagram) will not change drastically. What will change is how interesting the figure appears to the audience, and you want to benefit from that.