Maintaining upward compatibility with software written for the old system is important.
Literal
Old-system [for-用に] written software [with-との] upward-compatibility [object-を] maintain [nominalizer-のは] important is-[polite-です].
Software engineering note. ~用に ('for the use of X, made for X') — 用 + に marks intended use. 上位互換性 ('upward compatibility') is a technical compound — 上位 ('higher level') + 互換性 ('compatibility'). ~との is the composite particle 'with X + attributive.' 保つ ('to maintain, to preserve, to keep') is used for maintaining abstract qualities.