Moving /storage/sdcard0 from NAND to a removable MicroSD card: will it break anything?
Multiple things in this context:


apps might directly support using the external SD card as storage, check with their settings. This will save you some headaches, as it then works without "tricks". When changing to the new storage, apps might even offer to copy/move their data over, which would be the easiest thing for you to do.
As Gokul mentioned in his comments on your question: it's not as simple as just creating a symlink from the old storage to the new – after the next boot, that is most likely gone (which can be worked around by editing the boot image – or by facilitating init.d with a fitting script, if supported). You could however symlink each directory separately (e.g. /storage/sdcard0/foobar -&gt; /storage/external_sd/foobar, which should do even without root and, more importantly, survive a reboot without "additional tricks"
both external storages use VFAT, so no user permissions covered (VFAT doesn't support them), so "nothing to lose" in this context ;)