You should upgrade to 4G at your earliest convenience. It’s not an emergency because 3G modules should continue to work until 2022 but eventually you’ll need to, so the earlier the better.
Some customers have reported that their 3G signal has become weaker or less reliable over time and upgrading to 4G early resolved the issue. If you start experiencing connectivity problems with your 3G sled then definitely upgrade to 4G. But if you do it proactively then you can potentially avoid dealing with those communication issues.
You would have to notify us (send us a private message) when you’ve swapped the sleds and we will make the change on our end.