Migration are inevitable bad news, anticipate!
Migrations are a pain game for many customers. In the best case scenario everything stays the same for them. Why do they have to buy dozens of service hours? Why haven't they been warned at the time that the open source framework was chosen? How did you as a developer anticipate on a migration are how did you take care that a migration can be done fast and painless?
There a three ways to help customers to get a better feeling for the investment in a migration.
a. Inform the client that a caring owner (that is the client!) of a web system structurally reserves money and hours for maintenance. And that the other Web tools in the organisation (such as browsers) should be 'up to date'. A migration is inevitable in the near future. A piggy bank and a modern tools facilitates that migration. If a client is stuck up with closed source (proprietary) software the situation is far worse! Try not to convince too hard. As a developer you could pose a set of well-aimed questions. A smart customer creates his / her understanding with its own answers!
b. Promote the real benefits of the upcoming migration: spring cleaning of the 'old' (content and functionality), new features, speed and security, etc. Name advantages using the language of customers. Choose the right officer at the customer's organisation to opt for this: the boss. Try to tempt the boss by thinking towards his / her end customers (what may the new features of a migrated system specifically mean to those end customers?!). Give it a try!
These arguments are "no go's": "you need to be up to date" or "it's end of life, so you have to", "soon you might be hacked" or "The 9.0 version is nice, bro, chill!" Do not say these kind of things, customers get irritated pretty quickly, it's too nerdish and / or a developers point of view.
c. Anticipate on the inevitable migration in the interest of the customer, from your very first moves.
THINK WITH ALL YOU DO that a migration is upcoming. As a matter of fact and a way of life. What do you have to do or not do to facilitate a migration. How to anticipate on a migration to simply the actions and diminish the effort needed.
Migration isn't too bad after all: the organisation retains it's knowledge build up in various roles working with the CMS.