Depends on how much softer the wood is than the original... and how hard the original was.

Medium density (not super easy to crush between finger and thumb) is a fairly wide range of hardness... and anything in the range is fine. Soft balsa would cause a problem...

If hardening the balsa with CA... (or other glue, others can work) be sure you've done your sanding FIRST. Then after the glue is appld the grain will be raise a bit any you'll have to sand again. But if you apply the glue before rounding off the LE... you'll asnd off the hardened wood AND the sanding job will be very difficult.