Just keep the L first and topup another carton of M during 6.6. Even though u said it's few months away, eventually u still need to use L. If u got it at a nice price for L probably in the future u won't get back at this price again in the future.

My son used M for only a short period of time. Only used M for 3 months as the L size (9-14kg) overlap with M (6-11kg) and i find L is much comfier for my son when my son reached 9kg.