On the May Day Bank Holiday weekend more than 400,000 Cofunds users and £37bn of assets were moved across to the Aegon Platform.
Aegon bought Cofunds in 2016 for £140m and has been planning the integration of the two platforms ever since.
This was the third stage in the integration of Cofunds with Aegon, with the Investor Portfolio Service and institutional service having upgraded in December and March respectively.
Ahead of the switchover of assets as part of its integration of the Cofunds business, Aegon announced it would levy charges based on the value of the portfolio on the last working day of the month.
This differed from the approach taken by Cofunds, which calculated the fees daily and then charged investors once a month, and was criticised by Richard Cohen, adviser at Nsure Financial Planning in Worthing and user of the Cofunds platform.
Mr Cohen said the change in the way Aegon calculates fees could potentially mean a 30 times increase in the month on ongoing charges for any of his clients who invest on the last day of the month..
Alongside this change, Aegon is now also including the value of any residual cash balance held in a client account when it is calculating the value of the client portfolio, and levies the platform charge on any cash holding.
Cofunds did not charge for cash held by clients.
But a representative of Aegon said that, unlike Cofunds, Aegon also pays interest to clients on any residual cash in their investment account.
The rate of interest is higher than the level of charges on the platform, and so a spokesman for the provider claimed the vast majority of customers were better off as a result of the change.
A spokesman for Aegon added most clients only have a very small amount of cash in their accounts.
Residual balances can build up if a client is on a monthly savings plan and there is cash left over when they make investments.
Dividends received from investments can also add cash to a portfolio.
Aegon acquired Cofunds in August 2016, and recently completed an integration of the two businesses.
As FTAdviser has previously reported, the process of that integration left many advisers unable to service their clients needs.
Ian Lowes, managing director at Lowes Financial Management in Newcastle, who is a client of Aegon, said: "Clients will be marginally better off – Aegon currently pay 0.05 per cent below the Bank of England base rate so, 0.45 per cent.
"Given that the platform charge is significantly less, then even after tax, the effect will be marginally positive but not to the point that it is worth shouting about."
In contrast, rival provider Hargreaves Lansdown does not charge a fee for the cash balances in an account.
They do pay interest on deposits, with the interest rate varying depending on the level of cash in the account.