Is normal that the lower bearing is gone before the upper, all the weight is there. Usually is better to change both at the same time as the work to get there is the same and for the value of the bearing you make the job just ones. Hard to do or not is in depends your mechanical skills and the right tools also you didn’t mention which bike we are talking... I will suggest to read the Service Manual where for sure that job is described and then decide if you fill confident to do that..
Regards
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