Hiran
In practice, what you can do is to add /import the second mosaic dataset into the first one (via Add ‘table’); remove all ‘overviews’, and then re-do color-balancing (conduct statistics on this new MD, if necessary); and finally create overviews again.
In fact, your issue is common, especially, when working ‘non pre-processed’ raster datasets within Mosaic Dataset model.
Worth to mention here, if your ‘500 scenes’ raster datasets are not ‘pre-processed’ (that is, color-balancing and spatial rectification are not done by other remote sensing package like ERDAS/ ER Mapper or PCI), it is really challenging for you to produce nice color-balanced Mosaic Dataset with clear features at full resolution.