Square Enix ditched its in-house engine Luminous for Epic's Unreal Engine 4 part-way through the development of crossover action role-playing game Kingdom Hearts 3 - a move you'd expect to throw more than a few spanners in the works.

But according to its chief developer, the switch has been seamless, and development on the game is progressing smoothly.

Tai Yasue, co-director and game designer on Kingdom Hearts 3, the recently-released Kingdom Hearts 1.5 and the soon-to-be-released 2.5 HD Remixes, told Eurogamer in an interview that Unreal Engine 4 has impressed the team.

"The technology is moving forward a lot," he said. "That's something we've learned from the West, I think, in a big way. We're using Unreal Engine 4 right now, and that really has changed the way we make the game."

But development on Kingdom Hearts 3 began on Luminous before Square Enix shifted to Unreal 4, which begs the question: was there a problem with how Luminous was working out?

"There was nothing wrong with the Luminous engine at all," Yasue countered. "We decided that Unreal 4 was right for us. There was a huge network of people actually using it, we were communicating with the Japanese people at Epic a lot - it was like a complete product.

"It was easy to shift to, but at the same time though, there was a lot of stuff specific to Kingdom Hearts that we couldn't really do on Unreal 4 at first. So we had a lot of co-operation from Epic - they did a lot - at the same time we were doing a lot of customisation of the engine as well, to suit our needs."

These customisations have facilitated the likes of what the team refer to as the Kingdom Shader, Kingdom Hearts 3's paintbrush-like aesthetic.

"A lot of painstaking detail - the shadows for example, the really vivid colours, that sort of thing - we couldn't really do at the beginning, so we had to remake/customise the engine and add a lot of parameters."

Given that the game began life on Luminous, does that mean work on the initial teaser from E3 2013 had to be completely scrapped?

"We use the same graphical assets, so we didn't throw any of that away, and I guess a lot of the experience we got from making that for E3, that experience really translated to our development now," Yasue said.

"It was in our initial production stage, so I think we were learning a lot of stuff while using the Luminous engine."

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Legendary Square Enix designer Tetsuya Nomura recently stepped down as director of Square Enix's Final Fantasy 15 to focus on Kingdom Hearts 3.

"Nothing has really changed," Yasue said when asked whether the move had benefited development of the game.

"The development team is in Osaka, and Nomura-san didn't really do a lot of the detailed stuff, as he's really the sort of creative visionary.

"We communicate as much as we did previously, so our working relationship hasn't changed at all. We are speeding up things for development, obviously, on Kingdom Hearts 3 as we move along, but I don't think it has anything to do with Final Fantasy."

The recent glut of new Disney acquisitions evidently makes for some exciting new potential team-ups in the Kingdom Hearts universe - if we're thinking out loud it's no longer an impossibility for Cloud Strife, Spider-Man and Han Solo to embark on an adventure together along with Olaf from Frozen.

Has Disney made anything off-limits to the team at Square yet? "At our current stage, they have not said no," Yasui said.

"There's a lot to choose from, a lot of wonderful new IPs. No specifics but there are a lot of new IPs that we are considering and there's a lot on our table, and Disney hasn't said no - we don't have any trouble with that."

Kingdom Hearts 2.5 HD Remix launches in Europe tomorrow, but Square Enix is already looking ahead to the third instalment, due out PlayStation 4 and Xbox One sometime in the not too terribly distant future.

The idea is that 2.5 HD Remix will help prepare newcomers to the series for Kingdom Hearts 3's arrival, Yasue said.

"It's been 12 years since the original Kingdom Hearts came out in 2002; it's been a long while.

"We want new users to be able to understand the story before going into Kingdom Hearts 3, so I guess that was the plan - making Kingdom Hearts 3 but making it more accessible by making 1.5 and 2.5."