There's been studies on this. I couldn't find a study where a pacer or ICD failed.
A conclusions of a study done in 2007 should be reassuring:
"Conclusion: ECV for AF seems to be safe and effective in patients with implanted rhythm devices"
This was from a paper entitled: "External cardioversion of atrial fibrillation in patients with implanted pacemaker or cardioverter-defibrillator systems: a randomized comparison of monophasic and biphasic shock energy application"
You will be fine. You should ask your doc about the concern too, they can show you the type of ECV device they use which should be equiped with an external pacemaker just-in-caser.
My daughter had a duel chamber pacemaker and has had several cardioversions for A-Fib and she has NEVER had any problems with the pacemaker or leads following all of her cardioversions. They didn't need and replacements standing by or anything else.