We see in Star Trek III: The Search for Spock that Spock was able to perform a mind meld with McCoy to transfer his Katra to McCoy for later rejoining with Spock's body. This had an impact on McCoy's mind (which turned out to be quite humorous). My question is can a mind meld impact a person's psychology without transferring the Katra over e.g. could Spock perform a mind meld with McCoy and make him not like medicine and prefer astrology or something?
Yes, although not necessarily to the same extent
If you mean change their actual personality as Spock did to Bones, I can't think of an example. It's hard to say if Spock could make Bones love transporters and hate medice ... but if you mean impact their psychology to point of changing their behavor, Memory Alpha has several such examples:
In 2151, the v'tosh ka'tur Tolaris initiated a mind meld with Subcommander T'Pol against her will. Since Tolaris had not been properly trained in the use of his mental abilities, this meld caused T'Pol to develop Pa'nar Syndrome.
(ENT: "Fusion", "Stigma")
In 2268, on stardate 4385.3, Spock melded with Kirk, McCoy and Scott to ensure they believed the bullets fired at them from Virgil Earp, Morgan Earp, Wyatt Earp, and Doc Holliday (at the OK Corral) were unreal and, therefore, unable to harm them.
(TOS: "Spectre of the Gun")
In 2269, Spock melded with a sleeping Kirk, whispering a single word, "forget", to help Kirk forget his love for the female android Rayna Kapec.
(TOS: "Requiem for Methuselah")
The Spectre of the Gun example seems to suggest Spock could fool Bones into hating medicine.
Also, both Sarek and Captain Picard in the TNG episode Sarek, Sarek's deteriorating mental condition, Bendii syndrome, results in losing the control of his emotions, and he inadvertently broadcasts his emotions telepathically until the whole ship almost goes insane, either by arguing, insubordination, harassment, and brawls across the ship.
In a desperate measure to deal with the situation of Sarek negotiating peace talks, Sarek mind melds with Picard to let Picard lend his emotional control to Sarek temporarily. Due to this mind meld, Picard succumbs to some of Sarek's most primal emotions for a short time, until Sarek completed the negotiations, when he boards another starship to return to Vulcan.
You could say that Tuvok's meld with the Betazoid Lon Suder affected both their personalities:
An experimental Vulcan mind meld by Tuvok was carried out with the aim of helping Suder to control his impulses. While to some extent it was effective, the resulting trauma to the Vulcan's emotional systems resulted in him losing some of his own control, becoming dangerous and violent.
I'm not sure if this qualifies as a fundamental change in personality or just an alteration of the balance of components that were already present.
With regard to the scene in Act 4 of "Spectre of the Gun", what Spock did with the other members of the landing party was more than just a mind-meld. I had seen this done before, in "Dagger of the Mind", where he combined the meld---actually a quiet form of the Vulcan mind-fusion---with hypnotic suggestions of well-being, relaxation and weightless suspension, resulting in a calming effect on Van Gelder's mind. In "Spectre" he did the same thing---mind-meld combined with telepathic hypnosis, with spoken suggestions, each one worded differently for each member of the landing party but all with the same result: "The bullets are non-existent. Ignore them." And the only damage done in the shootout was to the wooden fence! Watching this electrifying procedure I got a whole new appreciation of what the Vulcans call "wuh tepul t'wuh kashek"---the power of the mind and what it could be capable of.