According to reports, the old calibrated gauge has been replaced on the lower meig with a new gauge, just at the put in below the dam. Handily, the new numbers are different so 4 is now a minimum level (compensation flow). This is equivalent to around 2.8 on the old gauge. This does not effect the SEPA gauge which is on the upper meig, above the reservoir.
It also appears that there has been some change in the boulders at the put in, around the dam stilling basing. This means the same amount of water doesn't 'look' as high below the dam as it once did. If you used to gauge the water level by the amount of concrete blocks covered, there will probably now be more concrete showingat a compensation flow.
There original forum post with the details can be found here.