I actually think 'leverage' is being used in a highly suspect way here in the EN text — probably by someone who knows it's a buzzword, but not what it actually means!

I feel pretty sure that the underlying sense here is simply 'to implement', and I believe 'mettre en œuvre' is a good way of rendering that idea in this particular instance.

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Note added at 16 mins (2009-10-14 16:31:58 GMT)
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Do note, Estelle, that your proposed translation of "pour permettre au client d’effectuer un changement opérationnel en douceur" would not, I think, be correct; they are in fact taling about using a combination of 'operational' and 'soft' changes — 2 quite different things (though only your wider context will enable you to tell just what those are). 'Operational' might, for example, be changes that have to be done 'live' / 'hot', while 'soft' changes might be ones that can be done in the background, off-line, and hence perhaps less mission-critical, for example.

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Note added at 16 mins (2009-10-14 16:32:32 GMT)
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Apologies for the typo: talking