The I-spread ("mid swap spread" or yield-yield spread) is a standlone measure of credit risk, a security against matched maturity vanilla swap rate. Consider a package in which the investor receives security's coupon and takes credit risk: interest rate exposure is delta-hedged with a matched-maturity swap.
The asset swap (ASW)-spread is a measure of total return, because coupon and principle cashflows are discounted at prevailing swap rate. Upfront principle, which may be large if the security price is far from par, means that the ASW-/I-spreads are often highly divergent.
Is there a closed form soln to convert from one to the other?