Why should you move to PediTree when you are used to Pedigree for DOS?
Only PediTree is still being developed, while Pedigree for DOS is not. Moreover, it has limitations when running under the new 64-bit Windows versions.
PediTree can use any database that was previously created or used in Pedigree for DOS. So, if you wish to start using PediTree, you can use your existing database(s) without change.
However, to gain full advantage from PediTree, you will need to have additional elements in the database definition. These are present in any database derived from the Elton55 design.
If your existing Pedigree database was derived from the Families or Census design, then you can easily move to one derived from Elton55. Export a GEDCOM 4 file from your existing database. Then open the Elton55 database and make a new (empty) database from it. Finally, import the GEDCOM file. See the section on GEDCOM for more information on this process.
You may also want to study this list of the advantages of PediTree.