CALGARY — Eastern gas distributors are crying foul over service changes proposed by TransCanada Corp. to its cross-country natural gas mainline that would limit shippers’ ability to renew delivery contracts, as the pipeline and power giant looks to switch portions of the long-haul system to carry oil.

TransCanada has begun soliciting shipper support to convert one of six tubes that comprise the mainline pipeline to oil service. The so-called Energy East project could move up to 850,000 barrels of western crude a day to Montreal and Quebec City refineries by 2017, and on to Irving Oil Ltd.’s plant in New Brunswick by 2018, TransCanada has said.

In March, the company told would-be shippers to expect less room on the 14,000-kilometre system for gas molecules in future as a result of the switch. Now, it wants to strip existing users of their ability to renew one-year delivery contracts on short notice, arguing longer-term commitments are needed to plan for new infrastructure development, including the oil conversion.

The change, proposed in filings made with the National Energy Board last week, is meant to ensure “we don’t get in a situation where [natural gas shippers] essentially tie up capacity just for one year and prevent it being made available for oil service for the remaining 29 years or whatever the oil business signs up for,” Stephen Clark, TransCanada’s senior vice-president, Canadian and Eastern U.S. gas pipelines, said in an interview Monday.

The move marks the start of another round in the bitter fight over the future of TransCanada’s ailing mainline. Round 1 wrapped up in March in a landmark case that ended with the NEB freezing long-distance tolls on the system at $1.42 per gigajoule for five years.

That decision came after tolls spiked on falling Alberta-to-Ontario shipments thanks to a rise in short-term delivery contracts underpinned by new production from shale gas basins located closer to consumers in eastern markets.

TransCanada has asked the board the revisit the outcome, proposing instead a 10¢ hike to the Alberta-to-Ontario toll, to $1.52 a gigajoule. Shippers currently pay $1.89 per gigajoule under an interim toll, Mr. Clark said.

With a review of that request still up in the air, TransCanada’s is effectively “putting the cart before the horse,” said Andrea Stass, a spokeswoman for Union Gas Ltd., which delivers natural gas to 1.4-million Ontario households and businesses.

The move to limit shippers’ ability to renew contracts “preempts the decision of the board and also the opportunity of those affected to protect their rights through a fair process,” she said.

Alberta Northeast Gas Ltd., a New England distributor, has likewise urged the energy regulator to either reject TransCanada’s proposal or subject it to review. Montreal-based Gaz Metro has also raised objections.

Under the existing regime, TransCanada said it may have to add incremental capacity that may be necessary for only one year, or else miss an opportunity to meet new service requests. “Neither of these outcomes would contribute to the efficient development of energy infrastructure, the rationalization of the system, or the public interest,” it told the board.

The Energy East project is in high demand as an Atlantic outlet for Alberta bitumen, with some of the sector’s biggest players, including oil sands giant Suncor Energy Inc., pledging support.

Mr. Clark said TransCanada needs “some certainty around how people want to use the system” as it tries to adapt a half-century old asset to a new competitive landscape.

“It’s not something that’s specific to the oil conversion,” he said. “It’s an important consideration, but it also applies to gas expansions as well.”