Last year my wife booked two 1st class tickets MEX-CDG one-way, economy return on LH for her sister and a friend using her UA miles. Initially she had book RT's in economy about 7 months out, but started looking for business or 1st class availability about 4 weeks out. She was able to change the outbound to 1st class 2 days before the departure. Availability out of MEX on LH or UA is pretty good outside of the summer months and Christmas holidays.