Conversion from Sibelius

Export to MusicXML on Sibelius, import that in MuseScore

Download the free trial of Sibelius Ultimate and install it and load and export your scores to MusicXML, seeing that it is a free trial for 30 days, this means it should be fully functional until the end of the trial period, which should be just long enough for you to convert your scores into MusicXML. And once you convert your scores, you will never have to worry about Sibelius ever again. However if I was you I would still hold on to your original sibelius scores as well as the converted MusicXML versions of your scores.

https://my.avid.com/get/sibelius-ultimate-trial

There is also Sibelius First which is free, however I do not know if it can load complex scores, it says it allows people to make simple scores,

https://my.avid.com/get/sibelius-first