The default treatment is to continue depreciating the original asset using all of the original information (including the original/old placed in service date), but associated with the new property. Then add a second asset with the new placed in service date to depreciate the 'extra' amount paid.
It is an ELECTION to combine it all as you said into one new asset using the new placed in service date. But that generally results in lower depreciation for the first years than if you do it the 'default' way.
If you do it the default way (depreciating it as two assets, which is usually is better), you need to finagle things. The easiest way is to just assign that old Asset Entry Worksheet to one of the properties (probably the new one), rather than splitting it between the two. It technically isn't correct, but I don't see any harm in it.
Otherwise, you need to have one of the Old assets for the old property for part of the year (with a disposition date). Then another one of the Old asset for the new property for part of the year (with the incorrect new placed in service date, so ProSeries only gives the partial year). Then next year change the placed in service to the old placed in service date.
EDIT: There is a section on the Asset Entry Worksheet for 1031 Exchanges for this. However, years ago I tried it and couldn't figure out if it actually did anything, so I've never used and just do things that way I described above.
