My current permission to stay, including re-entry permit, ends in October. Is there a way to change that (move it up) to December? The reason is that I will be out of the country in October and November.

Thanks.

No, but you can apply for the extension up to 30 days before your current permission to stay ends. In case of needing to travel immigration will often even allow it before the last 30 days.

The extension always starts from the date your current permission to stay ends, so by going early you don't lose any days and ensure you have enough time in case some documents are missing.

If your extension of stay ends in October you should be getting a new one now as it should always be done early and up to 30 days early is normal - if you show reason they will often allow it even earlier. Once you have the new extension of stay you can obtain a new re-entry permit to match it.

And just because you extend early does not mean you will get an earlier expiration date if you are concerned about that. The new extension of stay date will be one year from your current expiration date (i.e., no lost time by appling/extending early).