I remember playing old one decade ago and it certainly was different. Like some guards is been added, some persons who sat or idled are now pacing around fast.

I remember that on second warehouse level the vice president sat behind his table and i was able to steal values behind his neck and even read the paper he was reading without using violence. On GOG version he is running around his office, giving no opportunity to knock him out or sneak in unnoticed.

Only possible way is distraction. But approach is much more different, difficult and not so fun.

I don't think that the fan-made fixes included in the GOG version change anything that drastically. Which difficulty setting are you using? In the original game, the difficulty setting changed a lot of things. More enemies, different enemy behaviors, and even entire new objectives were added when playing on the harder difficulty settings. Perhaps you're just on a higher setting now than you were when you played it before?

But, I suppose it's possible there were some changes like this made to the GOG version, can anyone confirm?

it might be possible that he was meant to patrol around, but was not because of something now working properly in the original engine. as the GOG version uses the fixed version, the behavior might be different. but, as mentioned, many things can be playing a role in this - difficulty, and maybe false memories as well.