It should negotiate back down to ADSL.
But bear in mind that a 20 Magpackage may not make any difference if they are too far from exchange, say over 3 km line length.
What speed are they currently getting? What are their router stats?
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU BB => 2010: Orange 16 Meg LLU BB