Alliance Trust Savings customers will eventually be migrated onto Interactive Investor's own platform, the company has said.
Interactive Investor bought the Alliance Trust Savings platform at a discount price in October.
But it said the deal was still subject to regulatory approval which is expected to be done in the first half of 2019.
The direct-to-consumer investment platform paid £40m for Alliance Trust Savings, a price that included the company’s office in Dundee.
While II is a D2C platform, 30 per cent of ATS's clients are advised - a total of 32,500 accounts.
A spokesman for II said: "If regulatory approval is received, ATS customers will be migrated to Interactive Investor’s own platform over time and will become Interactive Investor customers."
She added that ATS and II were the largest flat fee retail investment platforms and there would be no changes to the current ATS charges, with customers getting "as good as, if not better" terms.
The company did not provide a timescale for when this would take place but a migration would come just four years after ATS started moving clients from its previous technology provider, Activebank, to GBST Composer.
Assets under administration on ATS fell by 7.6 per cent during 2018 to £14.6bn.
However, during 2018 the platform swung into profit, making £1.1m before tax compared with a loss of £19.3m the year before. Meanwhile revenue grew by 10 per cent to £29.6m.
Peter Docherty, chief executive of ATS, said: "2018 was a tremendous year for Alliance Trust Savings. It was a year where we invested heavily in the business and our people to deliver improved customer service and position ourselves for growth.
"The results announced today clearly demonstrate that this has worked. They also make it clear that there’s an obvious need for flat fees, in particular in the advised market, and we look forward to continuing to offer all our customers a genuinely different, and desired platform proposition, based on transparency and fairness."