The biggest problem when converting from Warsaw pact aircraft to western style aircraft is getting used to the different instruments.Couldn't really say - the Gripen is very digital, compared to the stuff like MiG-29s which are very analogue - so probably quite rapid conversion for those pilots used to modern hardware but maybe a struggle for the more old school pilots.
The worst being the attitude indicator (AKA the horizon ball). Because on a western plane you have a glass ball with a horizon bar fixed on the centre and inside the ball you have a floating horizon that moves relative to the bar (plane) to show you which way the plane is banking/diving/climbing. But on a Warsaw pact plane you have a glass ball with a horizon fixed inside it and a floating bar (plane). It may sound like a small difference but over the years there have been numerous incidents of pilots who are either used to or simply originally trained on Warsaw pact instruments turning their western aircraft the wrong way and sometimes even crashing as a result.
*EDIT*
This explains it far better than me:
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