As you know, in EPDM the serial number type definition can be attached to a text box designed on the data card. I hope you also would have known by now that there is no way to shift the serial number logics in real time when a document passes through its lifecycle (Ex: from development state to design state as you requested in a way).
Since serial number is directly linked to the data card definition and cannot be related or linked to the document lifecycle process, your requirement cannot be met directly but I am sure it is possible with below work around (though manual intervention is required).
Firstly, at admin configuration level, ensure to build two data cards (similar in look and feel) but store the data card definitions in different folders. Once done, have two different serial number definitions (ex; development & design serial number) and save the data cards.
Secondly, from user level, for the initial step, you will have to create the document in folders containing the data card definition with "development serial number". Your engineers can continue to use this serial number till the development continues.
Once the concept is frozen, you will have to have the engineer moving to the "Design" folder (basically different folder than the above) which contains the data card definition with "design serial number". Then have you engineer RMB (right mouse button) click on the serial number linked text box which will allow to generate a new serial number (this time design serial number).
Be careful, OOTB, it will not unfortunately restrict users to select this serial number option only once. The small issue here would be that engineers can continously keep RMB clicking any number of times. However, we can have a check on who did it but we end up doing post mortem tasks only. There is no proactive-preventive stop available in the present EPDM system.
The summary is that moving the file\document from one (development) folder to other (design) folder which are having two different serial number definitions in each of the file data cards linked in those folders, you will have the option to re-select a different serial number as two folders mentioned will have two serial numbers defined.
Hope the above information provides you an approach but I cannot say it's the best. If required you can try using EPDM API to automatically have this concept working.....!!!