Can the rotor resistance drop down from its design value?

What type of rotor ?

Helicopter rotor ?

Roto-rooter ?

Salient pole rotor ?

SSCIM rotor ?

What ? We can not read your mind !

Yes, it can. It depends on the designer QC/QA procedures and accuracy of their Measuring / testing instruments as well as Verification / calibration / tracibility of such instruments.

If its changed from a previously recorded value, using the same test methodology (preferebly by the same testers), .....then you have a problem Houston.

Design Value is the Ideal value which is being used.... When considering practical scenario, there is a high possibility of the resistance changing.

First all, resistance, in general case, depends from temperature. If you have calculated resistance of rotor's winding by using data from nameplate of your motor, you need to know that those data from nameplate are under full load conditions of your motor, so value of temperature under full load conditions of your motor isn't equal with value of temperature in conditions before starting of your motor. For example, after you have started your motor, you need to measure current of your motor immediately, because during work of your motor, windings will warm up until temperature stabilizes at rated value, which has a consequence increasing of resistance and decreasing of current in according to appropriate temperature conditions. After achieving appropriate work temperature of your motor, you need to turn off your motor from supply source and, after that, you can measure resistance of rotor's winding as you could compare it with appropriate value which you have calculated by using data from nameplate.
In order to get more precisely results of your measuring and in order to avoid a lot of different factors which have influence into results of your measuring, I would like to recommend you V/C method for measuring resistance in general case.
In your case, for measuring resistance of rotor's winding, you need to connect terminals of rotor's winding with DC supply source and to set value of DC current of your DC supply source at value which is equal with 10% of rated current of rotor's winding from nameplate of your motor. After that, you need to measure voltage between terminals of rotor by using multimeter and on that way measured value of voltage you need to share with value of current as you could to get the value of resistance of rotor's winding which should to be equal with value of resistance of rotor's winding which you have calculated by using data from nameplate of your motor. Of course, you need to measure resistance of rotor's winding for each phase and values of resistance of rotor's winding for each phase should to be almost equal.

Mr. Raymond
Its a salient pole rotor...

Generally it would be less than its design value, in order to allow for variations during construction.

WAY to little supplied info for others to provide you an educated response Amol.